Kingdom of Poland

It has been suggested that the early Slavic peoples and languages may have originated in the region of Polesia, which includes the area around the Belarus–Ukraine border, parts of Western Russia, and parts of far Eastern Poland.

The West Slavic and Lechitic peoples as well as any remaining minority clans on ancient Polish lands were organized into tribal units, of which the larger ones were later known as the Polish tribes; the names of many tribes are found on the list compiled by the anonymous Bavarian Geographer in the 9th century.

In the 9th and 10th centuries, these tribes gave rise to developed regions along the upper Vistula, the coast of the Baltic Sea and in Greater Poland. The latest tribal undertaking, in Greater Poland, resulted in the formation of a lasting political structure in the 10th century that became the state of Poland.

The tribe of the Polans (Polanie, lit. "people of the fields") in what is now Greater Poland gave rise to a tribal predecessor of the Polish state in the early part of the 10th century, with the Polans settling in the flatlands around the emerging strongholds of Giecz, Poznań, Gniezno and Ostrów Lednicki. Accelerated rebuilding of old tribal fortified settlements, construction of massive new ones and territorial expansion took place during the period ca. 920–950.

The Polish state developed from tribal roots in the second half of the century. According to the 12th-century chronicler Gallus Anonymus, the Polans were ruled at this time by the Piast dynasty. In existing sources from the 10th century, Piast ruler Mieszko I was first mentioned by Widukind of Corvey in his Res gestae saxonicae, a chronicle of events in Germany. Widukind reported that Mieszko's forces were twice defeated in 963 by the Veleti tribes acting in cooperation with the Saxon exile Wichmann the Younger. Under Mieszko's rule (ca. 960 to 992), his tribal state accepted Christianity and became the Polish state.

The viability of the Mieszko's emerging state was assured by the persistent territorial expansion of the early Piast rulers. Beginning with a very small area around Gniezno (before the town itself existed), the Piast expansion lasted throughout most of the 10th century and resulted in a territory approximating that of present-day Poland. The Polanie tribe conquered and merged with other Slavic tribes and first formed a tribal federation, then later a centralized state. After the addition of Lesser Poland, the country of the Vistulans, and of Silesia (both taken by Mieszko from the Czech state during the later part of the 10th century), Mieszko's state reached its mature form, including the main regions regarded as ethnically Polish. The Piast lands totaled about 250,000 km2 (96,526 sq mi) in area, with an approximate population of under one million.

Writing around 965 or 966 Ibrahim ibn Yaqub described the country of Mieszko, "the king of the North", as the most wide-ranging of the Slavic lands. Mieszko, the ruler of the Slavs, was also mentioned as such at that time by Widukind of Corvey in his Res gestae saxonicae. In its mature form, this state included the West Slavic lands between the Oder and Bug rivers and between the Baltic Sea and the Carpathian Mountains, including the economically crucial mouth areas of the Vistula and Oder rivers, as well as Lesser Poland and Silesia.

Before the adoption of Christianity in modern-day Poland, there were a number of different pagan tribes. Svetovid was among the most widespread pagan gods worshiped in Poland. Christianity arrived around the late 9th century, most likely around the time when the Vistulan tribe encountered the Christian rite in dealings with their neighbors, the Great Moravia (Bohemian) state.

"The Baptism of Poland" refers to the ceremony when the first ruler of the Polish state, Mieszko I, and much of his court converted to the Christian religion. Mieszko's wife Dobrawa of Bohemia, a zealous Christian, played a significant role in promoting Christianity in Poland, and might have had a significant influence on converting Mieszko himself. The Moravian cultural influence played a significant role in the spread of Christianity onto the Polish lands and the subsequent adoption of that religion. 

The exact place of Mieszko's baptism is disputed; Most historians argue that Gniezno or Poznań are the most likely sites. However, other historians have suggested alternative locations, such as Ostrów Lednicki, or even in German Regensburg. The date of Mieszko's baptism was 14 April 966, Holy Saturday.

After the normalization of relations with the Holy Roman Empire and Bohemia, Mieszko I returned to his plans to conquer the western part of Pomerania. On 21 September 967 the Polish-Bohemian troops prevailed in the decisive battle against the Wolinians led by Wichmann the Younger, which gave Mieszko control over the mouth of the Odra River. The German margraves had not opposed Mieszko's activities in Pomerania, perhaps even supported them; the death of the rebellious Wichmann, who succumbed to his wounds soon after the battle, may have been in line with their interests. A telling incident took place after the battle, a testimony to Mieszko's high standing among the Empire's dignitaries, just one year after his baptism: Widukind of Corvey reported that the dying Wichmann asked Mieszko to hand over Wichmann's weapons to Emperor Otto I, to whom Wichmann was related. For Mieszko the victory had to be a satisfying experience, especially in light of his past defeats inflicted by Wichmann.

Mieszko's state had a complex political relationship with the German Holy Roman Empire, as Mieszko was a "friend", ally and vassal of Holy Roman Emperor Otto I and paid him tribute from the western part of his lands. Mieszko fought wars with the Polabian Slavs, the Czechs, Margrave Gero of the Saxon Eastern March in 963–964 and Margrave Odo I of the Saxon Eastern March in 972 in the Battle of Cedynia. The victories over Wichmann and Odo allowed Mieszko to extend his Pomeranian possessions west to the vicinity of the Oder River and its mouth. After the death of Otto I, and then again after the death of Holy Roman Emperor Otto II, Mieszko supported Henry the Quarrelsome, a pretender to the imperial crown.

Probably in the early 980s Mieszko allied his country with Sweden against Denmark. The alliance was sealed with the marriage of Mieszko's daughter Świętosława with the Swedish king Erik. The content of the treaty is known from the traditional account—not entirely reliable, but originating directly from the Danish court—given by Adam of Bremen. In this text, probably as a result of confusion, he gives instead of Mieszko's name the name of his son Bolesław:

Mieszko decided on the alliance with Sweden probably in order to help protect his possessions in Pomerania from the Danish King Harald Bluetooth and his son Sweyn. They may have acted in cooperation with the Wolinian autonomous entity. The Danish were defeated ca. 991 and their ruler was expelled. The dynastic alliance with Sweden had probably affected the equipment and composition of Mieszko's troops. Perhaps at that time the Varangian warriors were recruited; their presence is indicated by archaeological excavations in the vicinity of Poznań.

Whether or not the German-Polish invasion of Bohemia actually happened, the friendly relations between the Czechs and the Poles came to an end. Bohemia resumed its earlier alliance with the Lutici, which, in 990, resulted in a war with Mieszko, who was supported by Empress Theophanu. Duke Boleslav II was probably the first one to attack. As a result of the conflict Silesia was taken over by Poland. However, the annexation of Silesia possibly took place around 985, because during this year the major Piast strongholds in Wrocław, Opole and Głogów were already being built.

The issue of the incorporation of Lesser Poland is also not completely resolved. Possibly Mieszko took the region before 990, which is indicated by the vague remark of Thietmar, who wrote of a country taken by Mieszko from Boleslav. In light of this theory, the conquest of Lesser Poland could be a reason for the war, or its first stage. Many historians suggested that the Czech rule over Lesser Poland was only nominal and likely limited to the indirect control of Kraków and perhaps a few other important centers. This theory is based on the lack of archaeological discoveries, which would indicate major building investments undertaken by the Bohemian state.

After its incorporation, Lesser Poland supposedly became the part of the country assigned to Mieszko's oldest son, Bolesław, which is indirectly indicated in the chronicle of Thietmar.

Bolesław I was a remarkable politician, strategist, and statesman. He not only turned Poland into a country comparable to older western monarchies, but he raised it to the front rank of European states. Bolesław conducted successful military campaigns in the west, south and east. He consolidated Polish lands and conquered territories outside the borders of modern-day Poland, including Slovakia, Moravia, Red Ruthenia, Meissen, Lusatia, and Bohemia. He was a powerful mediator in Central European affairs. Finally, as the culmination of his reign, in 1025 he had himself crowned King of Poland. He was the first Polish ruler to receive the title of rex (Latin: "king").

He was an able administrator who established the "Prince's Law" and built many forts, churches, monasteries and bridges. He introduced the first Polish monetary unit, the grzywna, divided into 240 denarii, and minted his own coinage. Bolesław I is widely considered one of Poland's most capable and accomplished Piast rulers.

In the summer of 1018, in one of his expeditions, Bolesław I captured Kiev, where he installed his son-in-law Sviatopolk I as ruler. According to legend, Bolesław chipped his sword when striking Kiev's Golden Gate. Later, in honor of this legend, a sword called Szczerbiec ("Jagged Sword") would become the coronation sword of Poland's kings.

One of the most important concerns of Bolesław's early reign was building up the Polish church. Bolesław cultivated Adalbert of Prague of the Slavník family, a well-connected Czech bishop in exile and missionary who was killed in 997 while on a mission in Prussia. Bolesław skillfully took advantage of his death: his martyrdom led to his elevation as patron saint of Poland and resulted in the creation of an independent Polish province of the Church with Radim Gaudentius as Archbishop of Gniezno. In the year 1000, the young Emperor Otto III came as a pilgrim to visit St. Adalbert's grave and lent his support to Bolesław during the Congress of Gniezno; the Gniezno Archdiocese and several subordinate dioceses were established on this occasion. The Polish ecclesiastical province effectively served as an essential anchor and an institution to fall back on for the Piast state, helping it to survive in the troubled centuries ahead.

In July 1002, Bolesław proceeded to a council with King Henry at the Kaiserpfalz of Merseburg in Saxony to deliberate on the enfeoffment of Meissen. As his claims were rejected, he left the royal court with disappointment. Moreover, an attempt was made on Bolesław's life, which he escaped only with the help of Duke Bernard of Saxony, the Nordgau margrave Henry of Schweinfurt and several friendly German nobles. While it is not known for sure if the attack had been ordered by Henry and the contemporary chronicler Thietmar of Merseburg denied any complicity, Bolesław believed this was the case. In any case Henry neither protected him, nor punished the assailants.

Bolesław started the feud when he had Strehla Castle in the Margraviate of Meissen set ablaze on his way back to Poland. The fighting began in late 1002, whereby the Polish ruler could rely on the support by Margrave Henry of Schweinfurt, whose expectations to become Bavarian duke had been disappointed by Henry. The German king further antagonised the Saxon nobility, when he forged an alliance with the pagan Luitici tribes against the Christian Polish realm on Easter 1003 in Quedlinburg. In turn, Henry had Margrave Gunzelin of Meissen, Eckard's brother, arrested and reached the commitment of several Saxon bishops.

After Bolesław had invaded Bohemia to depose Duke Boleslaus III, he was combated both by the Bohemian nobility and Boleslaus' brother Jaromír on the side of the German king. The fighting did not stop until Henry, with Bohemian and Lutici support, launched a campaign to Poznań, where a peace was concluded. As a result, Bolesław, unlike his ally Henry of Schweinfurt, refused to submit to King Henry, but had to give up his earlier conquests in Lusatia and Meissen.

In 1007, Bolesław, possibly preempting an attack by Henry, once again marched against the Luitici tribes. His campaign took him up to the gates of Magdeburg and he regained control of eastern Lusatia and Meissen. After several unsuccessful campaigns by the German king from 1010 onwards, another peace was agreed to in Merseburg in 1013. This time Bolesław kept eastern Lusatia and the Milceni lands around Bautzen as Imperial fiefs. He also received military aid from Henry for his intervention in the Kievan succession crisis. In return, Bolesław swore an oath of allegiance, promised to support Henry's bid for the crown of Holy Roman Emperor and aid him in his Italian campaigns. To confirm the alliance, Bolesław's son Mieszko II Lambert married the German noblewoman Richeza of Lotharingia, a distant relative of King Henry.

After Merseburg, Bolesław got entangled in the Kievan succession crisis backing his son-in-law Sviatopolk I against Henry's candidate Yaroslav the Wise. He thereby failed to support Henry in Italy and also refused to acknowledge Meissen and Lusatia as fiefs; he believed he held them independently of the Empire. To enforce Bolesław's submission, Henry had his son Mieszko II taken hostage and did not release him until 1014 following pressure from Saxon nobles.

Bolesław consistently refused to come before the German king. As a result, in 1015 Henry, supported by his pagan Liutician allies, launched another armed expedition against him. He attempted to cross into Greater Poland but was stopped by Bolesław's troops at Krosno on the Oder River. In 1017 Henry renewed his campaign, while Yaroslav attacked Poland from the eastern side. The emperor's troops besieged Niemcza in Silesia, however, with the help of outside reinforcements the city held out and Henry was eventually forced to retreat. The war spread over to Bohemia, where Mieszko's forces ravaged the lands and, while Bolesław again lost control over Kiev, peace efforts were resumed by the Saxon nobility.

The siege of Niemcza took place during three weeks in August 1017, in the last phase of the German–Polish War (1002–18), when the forces of the Emperor Henry II besieged the town of Niemcza controlled by the Polish ruler Bolesław I the Brave. Despite the aid of Bohemian and Lutici allies, the Imperial attack was ultimately unsuccessful, according to medieval chronicler Thietmar of Merseburg due to the arrival of reinforcements which managed to break into the city and the illness among the German forces. The failure of the siege marked the end of Henry's campaign in Poland and led the emperor to agree to the Peace of Bautzen in 1018, which left the eastern March of Lusatia and the Milceni lands (later Upper Lusatia) under Polish control.

Bolesław organized his first expedition east, to support his son-in-law Sviatopolk I of Kiev, in 1013, but the decisive engagements were to take place in 1018 after the peace of Budziszyn was already signed. At the request of Sviatopolk I, in what became known as the Kiev Expedition of 1018, the Polish duke sent an expedition to Kievan Rus' with an army of 2,000–5,000 Polish warriors, in addition to Thietmar's reported 1,000 Pechenegs, 300 German knights, and 500 Hungarian mercenaries. After collecting his forces during June, Bolesław led his troops to the border in July and on 23 July at the banks of the Bug River, near Wołyń, he defeated the forces of Yaroslav the Wise, Prince of Kiev, in what became known as the Battle of the River Bug. 

Yaroslav retreated north to Novgorod, opening the road to Kiev. The city, which suffered from fires caused by the Pecheneg siege, surrendered upon seeing the main Polish force on 14 August. The entering army, led by Bolesław, was ceremonially welcomed by the local archbishop and the family of Vladimir I of Kiev. According to popular legend Bolesław notched his sword (Szczerbiec) hitting the Golden Gate of Kiev. Although Sviatopolk lost the throne soon afterwards and lost his life the following year, during this campaign Poland re-annexed the Red Strongholds, later called Red Ruthenia, lost by Bolesław's father in 981.

The Battle of the River Bug, sometimes known as the Battle of Volhynia, was a battle that took place between the forces of Bolesław I the Brave of Poland and Yaroslav the Wise of Kievan Rus, during the Bolesław's Kiev Expedition. Yaroslav was defeated by the Polish duke. It was part of the war of succession following the death of Vladimir the Great in 1015. Boleslaw supported his son-in-law, Sviatopolk (known as Sviatopolk the Damned for his murder of his half-brothers Boris and Gleb), who was eventually defeated by Yaroslav.

While Yaroslav lost the battle, he was able to raise troops among the Novgorodians and eventually defeat his half-brother Sviatopolk and consolidate his position in Kiev, where he ruled over the golden age of Kievan Rus' until his death in 1054. Thus, in spite of this defeat at the hands of the Polish duke, Yaroslav became the greatest of the Kievan rulers.

Dissatisfaction with the process of Christianization, which had started after the baptism of Poland in 966, was one of the factors that led to the uprising. The Roman Catholic Church in Poland sustained substantial losses, with many churches and monasteries destroyed, and priests killed. The spread of the new Christian religion had been coupled with growth of the territories and central power of the king. In addition to anti-Christian sentiments, the rebellion showed elements of a peasant uprising against landowners and feudalism. Also present was a struggle for power between the king and some of the nobility. Anita Prazmowska notes, "Historians have concluded that in effect two overlapping revolutions had taken place simultaneously: a political and a pagan revolution."

The pagan reaction and related uprisings and rebellions of the time, coupled with foreign raids and invasions, threw the young Polish realm into chaos. Among the most devastating of the foreign contributions was a raid by Duke Bretislaus I of Bohemia in 1039, which pillaged Poland's first capital, Gniezno. According to some historians, the 1030s pagan uprising marks the end of the earliest period of Polish history, under the "First Piast Monarchy".

In 1047 Casimir, aided by his Kievan brother-in-law, started a war against Masovia and seized the land. It is probable that he also defeated Miecław's allies from Pomerania and attached Gdańsk to Poland. This secured his power in central Poland. Three years later, against the will of the Emperor, Casimir seized Bohemian-controlled Silesia, thus securing most of his father's domain. In 1054 in Quedlinburg, the Emperor ruled that Silesia was to remain in Poland in exchange for a yearly tribute of 117 kg. of silver and 7 kg. of gold.

At that time Casimir focused on internal matters. To strengthen his rule he re-created the bishopric in Kraków and Wrocław and erected the new Wawel Cathedral. During Casimir's rule heraldry was introduced into Poland and, unlike his predecessors, he promoted landed gentry over the drużyna as his base of power. One of his reforms was the introduction, to Poland, of a key element of feudalism: the granting of fiefdoms to his retinue of warriors, thus gradually transforming them into medieval knights.

Bolesław II the Bold is considered to have been one of the most capable of the Piast rulers. In 1075 he re-established the Archdiocese of Gniezno (consecrated in 1064) and founded the Diocese of Płock. He established Benedictine monasteries in Mogilno, Lubin and Wrocław. Bolesław II was also the first Polish monarch to produce his own coinage in quantity great enough to replace the foreign coins prevalent in the country during the reigns of the first Piast kings. He established royal mints in Kraków and Wrocław and reformed the coinage, which brought considerable revenue into the royal coffers. All these efforts had an enormous influence on the economic and cultural development of the country. According to the chronicler Gallus Anonymus, during his reign he was called largus ("the Generous" in English, "Szczodry" in Polish) as he founded many churches and monasteries throughout Poland.

After Bolesław's exile, the country found itself under the unstable rule of his younger brother Władysław I Herman (r. 1079–1102). Władysław was strongly dependent on Count Palatine Sieciech, an advisor from the ranks of the Polish nobility who acted much as the power behind the throne. When Władysław's two sons, Zbigniew and Bolesław, finally forced Władysław to remove his hated protégé, Poland was divided among the three of them from 1098, and after the father's death, from 1102 to 1106, it was divided between the two brothers.

After a power struggle, Bolesław III Wrymouth (r. 1102–1138) became the duke of Poland by defeating his half-brother Zbigniew in 1106–1107. Zbigniew had to leave the country, but received support from Holy Roman Emperor Henry V, who attacked Bolesław's Poland in 1109. Bolesław was able to defend his realm due to his military abilities, determination and alliances, and also because of a societal mobilisation across the social spectrum (see Battle of Głogów). Zbigniew, who later returned, died in mysterious circumstances, perhaps in the summer of 1113. Bolesław's other major achievement was the conquest of all of Mieszko I's Pomerania (of which the remaining eastern part had been lost by Poland from after the death of Mieszko II), a task begun by his father Władysław I Herman and completed by Bolesław around 1123. Szczecin was subdued in a bloody takeover and Western Pomerania up to Rügen, except for the directly incorporated southern part, became Bolesław's fief, to be ruled locally by Wartislaw I, the first duke of the Griffin dynasty.

The siege of Głogów was fought on 24 August 1109 at the Silesian town of Głogów, between the Kingdom of Poland and the Holy Roman Empire. Recorded by the medieval chronicler Gallus Anonymus, it is one of the most well known battles in Polish history. The Polish forces were led by Duke Bolesław III Wrymouth, while the Imperial forces were under the command of King Henry V of Germany. Bolesław was victorious.

Before he died, Bolesław III Wrymouth divided the country, in a limited sense, among four of his sons. He made complex arrangements intended to prevent fratricidal warfare and preserve the Polish state's formal unity, but after Bolesław's death, the implementation of the plan failed and a long period of fragmentation was ushered in. For nearly two centuries, the Piasts would spar with each other, the clergy, and the nobility for the control over the divided kingdom. The stability of the system was supposedly assured by the institution of the senior or high duke of Poland, based in Kraków and assigned to the special Seniorate Province that was not to be subdivided. Following his concept of seniorate, Bolesław divided the country into five principalities: Silesia, Greater Poland, Masovia, Sandomierz and Kraków. The first four provinces were given to his four sons, who became independent rulers. The fifth province, the Seniorate Province of Kraków, was to be added to the senior among the princes who, as the Grand Duke of Kraków, was the representative of the whole of Poland. This principle broke down already within the generation of Bolesław III's sons, when Władysław II the Exile, Bolesław IV the Curly, Mieszko III the Old and Casimir II the Just fought for power and territory in Poland, and in particular over the throne of Kraków. The external borders left by Bolesław III at his death closely resembled the borders left by Mieszko I.

In 1226, Duke Konrad I of Masovia invited the Teutonic Knights to help him fight the pagan, Baltic Old Prussians, who lived in a territory adjacent to his lands; substantial border warfare was taking place and Konrad's province was suffering from Prussian invasions. On the other hand, the Old Prussians themselves were at that time being subjected to increasingly forced, but largely ineffective Christianization efforts, including Northern Crusades sponsored by the papacy. The Teutonic Order soon overstepped their authority and moved beyond the area granted them by Konrad (Chełmno Land or Kulmerland). In the following decades, they conquered large areas along the Baltic Sea coast and established their own monastic state. As virtually all of the Western Baltic pagans became converted or exterminated (the Prussian conquests were completed by 1283), the Knights confronted Poland and Lithuania, then the last pagan state in Europe. Teutonic wars with Poland and Lithuania continued for most of the 14th and 15th centuries. The Teutonic state in Prussia, increasingly populated by German settlers beginning in the 13th century, but still retaining a majority Baltic population, had been claimed as a fief and protected by the popes and Holy Roman Emperors.

The Mongol Invasion of Poland from late 1240 to 1241 culminated in the Battle of Legnica, where the Mongols defeated an alliance which included forces from fragmented Poland and their allies, led by Henry II the Pious, the Duke of Silesia. The first invasion's intention was to secure the flank of the main Mongolian army attacking the Kingdom of Hungary. The Mongols neutralized any potential help to King Béla IV being provided by the Poles or any military orders.

The second Mongol invasion of Poland was carried out by general Boroldai (Burundai) in 1259–1260. During this invasion the cities of Sandomierz, Kraków, Lublin, Zawichost, and Bytom were sacked by Mongols for the second time.

The third Mongol invasion of Poland was carried out by Talabuga Khan and Nogai Khan in 1287–1288. As in the second invasion, its purpose was to loot Lesser Poland, and to prevent Duke Leszek II the Black from interfering in Hungarian and Ruthenian affairs. The invasion was also part of the hostilities between Poland and Ruthenia; in 1281, the Poles had defeated a Mongol force near Goslicz which had entered Duke Leszek's territory in support of Lev I.

Compared to the first two invasions, the raid of 1287–88 was short and much less devastating. The Mongols did not capture any significant cities or castles and lost a significant number of men. They also took fewer prisoners and loot than in the previous invasions.

In 1295, Przemysł II of Greater Poland became the first Piast duke crowned as King of Poland since Bolesław II, but he ruled over only a part of the territory of Poland (including Gdańsk Pomerania from 1294) and was assassinated soon after his coronation. A more extensive unification of Polish lands was accomplished by a foreign ruler, Václav II of Bohemia of the Přemyslid dynasty, who married Przemysł's daughter Richeza and became King of Poland in 1300.

The city of Danzig (Gdańsk) was captured by the State of the Teutonic Order on 13 November 1308, resulting in a massacre of its inhabitants and marking the beginning of tensions between Poland and the Teutonic Order. Originally the knights moved into the fortress as an ally of Poland against the Margraviate of Brandenburg. However, after disputes over the control of the city between the Order and the King of Poland arose, the knights murdered a number of citizens within the city and took it as their own. Thus the event is also known as Gdańsk massacre or Gdańsk slaughter (rzeź Gdańska). Though in the past a matter of debate among historians, a consensus has been established that many people were murdered and a considerable part of the town was destroyed in the context of the takeover. In the aftermath of the takeover, the order seized all of Pomerelia (Gdańsk Pomerania) and bought up the supposed Brandenburgian claims to the region in the Treaty of Soldin (1309). The conflict with Poland was temporarily settled in the Treaty of Kalisz/Kalisch (1343). The town was returned to Poland in the Peace of Toruń/Thorn in 1466.

Władysław I inherited a small portion of his father's domain, but his dominion grew as some of his brothers died young. He unsuccessfully tried to incorporate the Duchy of Krakow (the Seniorate Province) in 1289, following the death of his half-brother Leszek II the Black and the withdrawal from contention of his ally Bolesław II of Masovia. After a period in exile during the rule of Wenceslaus II, Władysław regained several duchies and then Krakow in 1306 when Wenceslaus III was murdered. He temporarily took control of part of Greater Poland after the death of his ally Przemysł II, lost it, and then subsequently regained it.

Władysław was a skilled military leader, but also an administrator; he conquered Gdańsk Pomerania, and left it to familial governors. For the defense of this territory, he turned to the Teutonic Knights, who then demanded an exorbitant sum or the land itself as an alternative. This led to an extended battle with the Knights, which was not resolved after either a papal trial or Władysław's own death. Perhaps his greatest achievement was gaining papal permission to be crowned king of Poland in 1320, which occurred for the first time at Wawel Cathedral in Krakow. Władysław died in 1333 and was succeeded by his son, Casimir III the Great.

The Polish–Teutonic War (1326–1332) was the war between the Kingdom of Poland and the State of the Teutonic Order over Pomerelia, fought from 1326 to 1332.

A lengthy, hard-fought battle ensued that lasted from sunrise until 3:00 p.m. the same day. The armies were fairly evenly matched, and the deadlock was only broken when a horse carrying the marshal's banner was pierced by a spear and the Teutonic army saw the banner fall, assuming that their leader had fallen, and began to flee the battle.

The Polish forces took advantage of the fleeing Teutons and struck hard, turning the tide of the battle in their favor. By the end of the battle, Władysław and his son Casimir III of Poland had 56 Teutonic knights in their custody, along with Altenburg. Instead of ransoming the knights, Władysław ordered that they be executed on the spot. He spared the marshal and a handful of wealthy Teutonic noblemen, whom he intended to ransom.

An army of Teutonic knights was dispatched from Prussia to relieve the forces at Płowce and were fast approaching the battle. The exhausted Polish troops engaged in another hard-fought battle, continuing until nightfall, but the Polish forces were eventually crushed. Altenburg was released after he was found chained to a wagon. The marshal ordered that all Polish captives were to be executed.

Casimir inherited a kingdom weakened by war and made it prosperous and wealthy. He reformed the Polish army and doubled the size of the kingdom. He reformed the judicial system and introduced a legal code, gaining the title "the Polish Justinian". Casimir built extensively and founded the Jagiellonian University (back then simply called the University of Krakow), the oldest Polish university and one of the oldest in the world. He also confirmed privileges and protections previously granted to Jews and encouraged them to settle in Poland in great numbers.

Casimir left no sons. When he died in 1370 from an injury received while hunting, his nephew, King Louis I of Hungary, succeeded him as king of Poland in personal union with Hungary.

The Treaty formally concluded the Polish-Teutonic War which began in 1326/1327. It put an end to the long-running diplomatic clashes over Pomerelia including Gdańsk that ensued the War, an area that the Teutonic Order had in its possession since 1308, and which it viewed as its legal property since the conclusion of the Treaty of Soldin (1309) with the Margraves of Brandenburg. In the Treaty of Kalisz, King Casimir III, undertook in the future to raise no claims on Pomerelia as well as Chełmno Land and Michałów Land. In exchange, King Casimir III regained Kuyavia and Dobrzyń Land, which had been conquered by the Teutonic Order between 1329 and 1332. The peace agreement also confirmed seven cities: Poznań and Kalisz in Greater Poland, Włocławek and Brześć Kujawski in Kuyavia, as well as Kraków, Sandomierz and Nowy Sącz in Lesser Poland.

As a result, while Pomerelia remained a subject of contention, the treaty was followed by 66 years of peace between the Kingdom of Poland and the Teutonic Order, until the conflict erupted again in the Polish–Lithuanian–Teutonic War of 1409.

Casimir ensured stability and great prospects for the future of the country. He established the Corona Regni Poloniae – the Crown of the Polish Kingdom, which certified the existence of the Polish lands independently from the monarch. Prior to that, the lands were only the property of the Piast dynasty.

At the Sejm in Wiślica, on 11 March 1347, Casimir introduced reforms to the Polish judicial system and sanctioned civil and criminal codes for Great and Lesser Poland, earning the title "the Polish Justinian".

In the mid-14th century, King Casimir III the Great realised that the nation needed a class of educated people, especially lawyers, who could arrange a better set of the country's laws and administer the courts and offices. His efforts to found an institution of higher learning in Poland were rewarded when Pope Urban V granted him permission to set up a university in Kraków. A royal charter of foundation was issued on 12 May 1364, and a simultaneous document was issued by the City Council granting privileges to the Studium Generale.

The Congress of Kraków (Polish: Zjazd krakowski) was a meeting of monarchs initiated by King Casimir III the Great of Poland and held in Kraków (Cracow) around September 22–27, 1364. The pretext for calling the meeting was very likely a proposed anti-Turkish crusade, but the Congress was actually concerned mostly with European diplomacy issues, of which preeminent were peaceful relations and the balance of power in central Europe and negotiating a common response to the Turkish threat through the project of a central European league of states.

The participants - guests of the Polish king were Charles IV, Holy Roman Emperor, King Louis I of Hungary, King Valdemar IV of Denmark, King Peter I of Cyprus, Siemowit III of Masovia, Bolko II of Świdnica, Władysław Opolczyk, Rudolf IV, Duke of Austria, Bogislaw V, Duke of Pomerania, Casimir IV, Duke of Pomerania, Otto V, Duke of Bavaria and Louis VI the Roman. The Congress, which took place in lavish surroundings, intended as a manifestation of the Polish king's power and wealth, echoed throughout Europe.

The first union of Hungary and Poland came about when the last Piast king of Poland, Casimir III, designated his sororal nephew, the Angevin king Louis I of Hungary, as his heir presumptive by the Privilege of Buda. Upon the death of Casimir, who left no legitimate sons, Louis ascended the Polish throne virtually unopposed. The Polish nobility welcomed his accession, rightly believing that Louis would be an absentee king who would not take much interest in Polish affairs. He sent his mother Elizabeth, sister of Casimir III, to govern Poland as regent. Louis probably considered himself first and foremost king of Hungary; he visited his northern kingdom three times and spent there a couple of months altogether. Negotiations with the Polish nobility frequently took place in Hungary. Hungarians themselves were unpopular in Poland, as was the king's Polish mother who governed the kingdom. In 1376, circa 160 Hungarians in her retinue were massacred in Kraków and the queen returned to Hungary disgraced. Louis replaced her with their relative, Vladislaus II of Opole.

Louis' death in 1382, without a male heir, left a power vacuum (interregnum). Although the Privilege of Koszyce stipulated that one of his daughters would succeed him on the Polish throne, Louis' selection of his daughter Mary proved controversial, as her husband, Sigismund of Luxembourg, was not popular in Poland. The different factions in Poland could not agree on the succession, and a conflict erupted. The faction gathered around the Grzymała clan supported Sigismund, while the Nałęcz clan instead favored the Duke of Masovia, Siemowit IV.

As the clans in Greater Poland warred, those in Lesser Poland succeeded in gathering support for a different solution. In 1384, Louis' 10-year-old daughter Jadwiga was crowned King of Poland, upon the condition that the Polish-Hungarian Union was dissolved. Her coronation marked the end of most civil war hostilities; Norman Davies notes that "the disappointed candidates battled each other's candidacy into oblivion". As Jadwiga's fiance, William, Duke of Austria, was also unpopular in Poland, the Lesser Poland faction succeeded in arranging for her to marry Władysław Jagiełło (Jogaila), Grand Duke of Lithuania, in 1386. Jagiełło had just emerged victorious from a civil war in Lithuania. The war is said to have been bloody; Davies writes of "much slaughter", and Sobczak notes that "entire clans perished in it".

In a strict sense, the Union of Krewo comprised a set of prenuptial promises made at Kreva Castle on 14 August 1385 by Jogaila, Grand Duke of Lithuania, in regard to his prospective marriage to the underage reigning Queen Jadwiga of Poland.

After the 1385 negotiations, Jogaila, Grand Duke of Lithuania converted to Christianity, to the underage Queen Jadwiga, and was crowned King of Poland in 1386.

The union proved a decisive moment in the histories of Poland and Lithuania; it marked the beginning of four centuries of shared history of the two polities. By 1569 the Polish–Lithuanian union had developed into a new state, the Polish–Lithuanian Commonwealth, which lasted until the Third Partition of Poland in 1795.