Poland during the Jagiellonian dynasty

In 1385, the Union of Krewo was signed between Queen Jadwiga of Poland and Jogaila, the Grand Duke of Lithuania, the ruler of the last pagan state in Europe. The act arranged for Jogaila's baptism and the couple's marriage, which established the beginning of the Polish–Lithuanian union. After Jogaila's baptism, he was known in Poland by his baptismal name Władysław and the Polish version of his Lithuanian name, Jagiełło. The union strengthened both nations in their shared opposition to the Teutonic Knights and the growing threat of the Grand Duchy of Moscow.

The population of the Grand Duchy's enlarged territory was accordingly heavily Ruthenian and Eastern Orthodox. The territorial expansion led to a confrontation between Lithuania and the Grand Duchy of Moscow, which found itself emerging from the Tatar rule and itself in a process of expansion. Uniquely in Europe, the union connected two states geographically located on the opposite sides of the great civilizational divide between the Western Christian or Latin world, and the Eastern Christian or Byzantine world.

The Lithuanian Civil War of 1389–1392 was the second civil conflict between Jogaila, King of Poland and Grand Duke of Lithuania, and his cousin Vytautas. At issue was control of the Grand Duchy of Lithuania, then the largest state in Europe. Jogaila had been crowned King of Poland in 1386; he installed his brother Skirgaila as ruler of Lithuania. Skirgaila proved unpopular and Vytautas attempted to depose him. When his first attempt to take the capital city of Vilnius failed, Vytautas forged an alliance with the Teutonic Knights, their common enemy – just as both cousins had done during the Lithuanian Civil War between 1381 and 1384. Vytautas and the Knights unsuccessfully besieged Vilnius in 1390. Over the next two years it became clear that neither side could achieve a quick victory, and Jogaila proposed a compromise: Vytautas would become Grand Duke and Jogaila would remain Superior Duke. This proposal was formalized in the Ostrów Agreement of 1392, and Vytautas turned against the Knights. He went on to reign as Grand Duke of Lithuania for 38 years, and the cousins remained at peace.

Samogitian uprisings refer to two uprisings by the Samogitians against the Teutonic Knights in 1401–1404 and 1409. Samogitia was granted to the Teutonic Knights by Vytautas the Great, Grand Duke of Lithuania, several times in order to enlist Knights' support for his other military affairs. The local population resisted Teutonic rule and asked Vytautas to protect them. The first uprising was unsuccessful and Vytautas had to reconfirm his previous promises to transfer Samogitia in the Peace of Raciąż. The second uprising provoked the Knights to declare war on Poland. Hostilities escalated and resulted in the Battle of Grunwald (1410), one of the biggest battles of medieval Europe. The Knights were soundly defeated by the joint Polish–Lithuanian forces, but Vytautas and Jogaila, King of Poland, were unable to capitalize on their victory. Conflicts regarding Samogitia, both diplomatic and military, dragged until the Treaty of Melno (1422).

In December 1408, Władysław and Vytautas held strategic talks in Navahrudak Castle, where they decided to foment a Samogitian uprising against Teutonic rule to draw German forces away from Pomerelia. Władysław promised to repay Vytautas for his support by restoring Samogitia to Lithuania in any future peace treaty. The uprising, which began in May 1409, at first provoked little reaction from the Knights, who had not yet consolidated their rule in Samogitia by building castles; but by June their diplomats were busy lobbying Władysław's court at Oborniki, warning his nobles against Polish involvement in a war between Lithuania and the Order. Władysław, however, bypassed his nobles and informed new Grand Master Ulrich von Jungingen that if the Knights acted to suppress Samogitia, Poland would intervene. This stung the Order into issuing a declaration of war against Poland on 6 August, which Władysław received on 14 August in Nowy Korczyn.

The Battle of Grunwald, Battle of Žalgiris or First Battle of Tannenberg was fought on 15 July 1410 during the Polish–Lithuanian–Teutonic War. The alliance of the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania, led respectively by King Władysław II Jagiełło (Jogaila) and Grand Duke Vytautas, decisively defeated the German Teutonic Order, led by Grand Master Ulrich von Jungingen. Most of the Teutonic Order's leadership were killed or taken prisoner. Although defeated, the Teutonic Order withstood the siege of the Malbork Castle and suffered minimal territorial losses at the Peace of Thorn (1411), with other territorial disputes continuing until the Treaty of Melno in 1422. The order, however, never recovered their former power, and the financial burden of war reparations caused internal conflicts and an economic downturn in the lands controlled by them. The battle shifted the balance of power in Central and Eastern Europe and marked the rise of the Polish–Lithuanian union as the dominant regional political and military force.The battle was one of the largest in medieval Europe. The battle is viewed as one of the most important victories in the histories of Poland and Lithuania. It is also commemorated in Ukraine and Belarus. It has been used as a source of romantic legends and national pride, becoming a larger symbol of struggle against foreign invaders. During the 20th century, the battle was used in Nazi German and Soviet propaganda campaigns.

The Union of Horodło or Pact of Horodło was a set of three acts signed in the town of Horodło on 2 October 1413. The first act was signed by Władysław II Jagiełło, King of Poland, and Vytautas, Grand Duke of Lithuania. The second and third acts were composed by the Polish nobility (szlachta) and Lithuanian boyars, respectively. The union amended the earlier Polish–Lithuanian unions of Krewo and Vilnius–Radom. Politically, Lithuania received more autonomy as, after the death of Vytautas, the Lithuanian nobles could choose another Grand Duke instead of passing the title to Władysław II Jagiełło or his heir. However, culturally, Lithuania and Poland grew closer. Lithuania adopted Polish institutions of castellans and voivodes. Catholic Lithuanian nobles and church officials were granted equal rights with the Polish nobles and clergy. Forty-seven selected Lithuanian nobles were adopted by Polish families and granted Polish coats of arm. Thus the union signified the beginnings of the Polonization of Lithuanian culture and the rise of the Lithuanian nobility.

In the summer of 1414, armies of King Jogaila and Grand Duke Vytautas invaded Prussia, ruled by the monastic state. They advanced through Osterode (Ostróda) into Warmia, plundering villages and burning crops. The Teutonic Knights chose to concentrate their defensive efforts in Culmerland (Chełmno Land). The Knights remained in their castles and refused an open battle after realizing the superiority of Polish and Lithuanian forces in a potential pitched battle. Küchmeister deployed scorched earth tactics hoping to deprive invading armies of food and supplies. This tactic later resulted in a famine and plague in the region. The invaders themselves were not able or willing to seek a decisive military victory via lengthy sieges of Teutonic castles. Papal legate William of Lausanne proposed resolving the conflict through diplomacy and a two-year truce was signed in Strasburg (now Brodnica) in October. Jogaila and Vytautas agreed to present their case to the Council of Constance. However, the territorial disputes were not resolved until the Treaty of Melno in 1422.

In July 1422, the Emperor Sigismund and the Teutonic Knights devoted resources to a war against the Hussites, who attacked and devastated large parts of Germany. The pope called for strong measure to 'get rid of this plague'. Vytautas and Jogaila used the preoccupation with defense against the Hussite raids by attacking Prussia and the Order. Teutonic Grand Master Michael Küchmeister von Sternberg was forced to resign in March. His successor Paul von Rusdorf released most of the hired mercenaries; the Order was left with very few soldiers to defend itself. Joint Polish and Lithuanian forces marched north to Osterode, Teutonic forces retreated to Löbau. When it became clear that siege engines would not arrive, Jogaila ordered an advance towards the Order's fortified capital of Marienburg. His army captured Riesenburg and pillaged surrounding villages. Heading south to Chełmno Land, the Poles and Lithuanians then captured Gollub, but failed to take Schönsee. Jogaila decided to end the war quickly before the overwhelmed Prussian troops of the Order could receive reinforcements from the Holy Roman Empire that Paul von Rusdorf had requested. A truce was signed on September 17, 1422, and the war concluded ten days later with the Treaty of Melno. This ended the territorial disputes and fights between Lithuania and the Teutonic Knights. Poland, however, resumed fighting with the Order once again in 1431–1435 when the Order supported Švitrigaila and not Polish-backed Sigismund Kęstutaitis as the successor of Vytautas.

The war broke out after Teutonic Grand Master Paul von Rusdorf signed the Treaty of Christmemel, creating an alliance with Švitrigaila, who was waging a civil war against his brother Polish King Jogaila (Władysław Jagiełło) for the throne of the Grand Duchy of Lithuania. Sigismund of Luxemburg made a commitment to the Teutonic Order in an effort to break the Polish–Lithuanian union.

In 1431, while the main Polish forces were involved in Lutsk in Volhynia, the Teutonic Knights invaded Poland. Finding little opposition, the Knights ravaged Dobrzyń Land, taking the town of Nieszawa, and tried to move on to the Kuyavia and Krajna regions. However, the Teutonic army was defeated on 13 September 1431 in the Battle of Dąbki, near Nakel (Nakło nad Notecią). In September a two-year truce was signed among Poland, Lithuania and the Teutonic Knights at Staryi Chortoryisk.

In June 1433 Poland allied itself with the Czech Hussites in order to stop the Teutonic Order from sending secret support to Švitrigaila via its Livonian branch. The Teutonic Knights had supported the Pope and Holy Roman Emperor Sigismund against the heretic Hussites during the Hussite Wars. Czech forces under Jan Čapek of Sány were granted safe passage through Poland for their last and largest "beautiful ride." The Polish forces were also supported by Pomeranian Duke Bogusław IX of the Duchy of Stolp (Słupsk). In addition, the Moldavians, whose ruler Iliaş had been replaced by the more pro-Polish Stephen II, had joined the Polish alliance. For four months the Hussite army, including forces led by Feodor Ostrogski, ravaged Teutonic territories in Neumark, Pomerania, and western Prussia. First they unsuccessfully besieged Konitz (Chojnice) for six weeks, then moved north to Schwetz (Świecie) and Danzig (Gdańsk). They captured several towns and castles, including Dirschau (Tczew) on the Vistula River (29 August 1433). Despite their failed siege of Danzig, the Hussites reached the Baltic Sea near Oliwa at the beginning of September and celebrated their "beautiful ride" by symbolically filing their bottles with water from the sea. Returning to the south via Starogard Gdański, the expedition occupied a castle in the frontier settlement of Nowy Jasiniec.

The Battle of Wiłkomierz took place on September 1, 1435, near Ukmergė in the Grand Duchy of Lithuania. With the help of military units from the Kingdom of Poland, the forces of Grand Duke Sigismund Kęstutaitis soundly defeated Švitrigaila and his Livonian allies. The battle was a decisive engagement of the Lithuanian Civil War (1432–1438). Švitrigaila lost most of his supporters and withdrew to southern Grand Duchy; he was slowly pushed out and eventually made peace. The damage inflicted upon the Livonian Order has been compared to the damage of Battle of Grunwald upon the Teutonic Order. It was fundamentally weakened and ceased to play a major role in Lithuanian affairs. The battle can be seen as the final engagement of the Lithuanian Crusade.

The battle reduced the power of the Livonian Order as its army was defeated, its Grand Master killed and of its many senior officers killed or taken prisoner. The damage to the Livonian Order caused by the battle is often compared to the consequences that the Battle of Grunwald (1410) had on the Teutonic Knights. The peace treaty was signed on December 31, 1435 in Brześć Kujawski. The Teutonic and Livonian Orders promised not to interfere in internal matters of Lithuania or Poland. According to the treaty, even the Pope or the Holy Roman Emperor could not force the Orders to violate the accords. That provision was meant particularly against Sigismund, Holy Roman Emperor, and his attempts in playing Poland and Teutonic Order against each other.

Casimir IV was one of the most active Polish-Lithuanian rulers, under whom Poland, by defeating the Teutonic Knights in the Thirteen Years' War recovered Pomerania, and the Jagiellonian dynasty became one of the leading royal houses in Europe.

The great triumph of his reign was bringing Prussia under Polish rule. The rule of Casimir corresponded to the age of "new monarchies" in western Europe. By the 15th century Poland had narrowed the distance separating it from western Europe and become a significant factor in international relations. The demand for raw materials and semi-finished goods stimulated trade, producing a positive balance, and contributed to the growth of crafts and mining in the entire country. He was a recipient of the English Order of the Garter (KG), the highest order of chivalry and the most prestigious honour in England.

The Thirteen Years' War was a conflict fought in 1454–1466 between the Prussian Confederation, allied with the Crown of the Kingdom of Poland, and the State of the Teutonic Order. The war began as an uprising by Prussian cities and local nobility to win independence from the Teutonic Knights. In 1454 Casimir IV married Elisabeth of Habsburg and the Prussian Confederation asked Poland's King Casimir IV Jagiellon for help and offered to accept the king as protector instead of the Teutonic Order. When the King assented, war broke out between supporters of the Prussian Confederation, backed by Poland, and backers of government by the Teutonic Knights. The Thirteen Years' War ended in the victory of the Prussian Confederation and Poland and in the Second Peace of Thorn (1466). This was soon followed by the War of the Priests (1467–1479), a drawn-out dispute over the independence of the Prussian Prince-Bishopric of Warmia (Ermland), in which the Knights also sought revision of the Peace of Thorn.

The War of the Priests was a conflict in the Polish province of Warmia between the King of Poland Casimir IV and Nicolaus von Tüngen, the new bishop of Warmia chosen – without the king's approval – by the Warmian chapter. The latter was supported by the Teutonic Knights, by this point vassals of Poland, who were seeking a revision of the recently signed Second Peace of Toruń.

In 1477 Martin von Wetzhausen, the new grand master of the order refused to make his oath of fealty to the Polish king and invaded Warmia, taking Chełm and Starogard Chełminski. In response, in 1478, Polish forces of King Casimir IV intervened militarily, besieging Braniewo. Under the command of Jan Biały and Piotr Dunin, the Polish forces occupied several cities in Warmia and Pomesania. The Teutonic Knights' military operations were hampered by the refusal of the Prussian Estates to support them. Tüngen was forced to flee to Königsberg (Królewiec). At the same time, in April 1479, the Polish and Hungarian kings came to an agreement and Corvinus withdrew from the anti-Polish alliance. As a consequence by July 1479, both Tüngen and the grand master were forced to pay homage to the Polish king.

The influence of the Jagiellonian dynasty in Central Europe rose during the 15th century. In 1471, Casimir's son Władysław became king of Bohemia, and in 1490, also of Hungary. The southern and eastern outskirts of Poland and Lithuania became threatened by Turkish invasions beginning in the late 15th century. Moldavia's involvement with Poland goes back to 1387, when Petru I, Hospodar of Moldavia sought protection against the Hungarians by paying homage to King Władysław II Jagiełło in Lviv. This move gave Poland access to Black Sea ports. In 1485, King Casimir undertook an expedition into Moldavia after its seaports were overtaken by the Ottoman Turks. The Turkish-controlled Crimean Tatars raided the eastern territories in 1482 and 1487 until they were confronted by King John Albert, Casimir's son and successor.

Poland was attacked in 1487–1491 by remnants of the Golden Horde that invaded Poland as far as Lublin before being beaten at Zaslavl. In 1497, King John Albert made an attempt to resolve the Turkish problem militarily, but his efforts were unsuccessful; he was unable to secure effective participation in the war by his brothers, King Vladislas (Władysław) II of Bohemia and Hungary and Alexander, the Grand Duke of Lithuania, and he also faced resistance on the part of Stephen the Great, the ruler of Moldavia. More destructive Tatar raids instigated by the Ottoman Empire took place in 1498, 1499 and 1500. Diplomatic peace efforts initiated by John Albert were finalized after the king's death in 1503. They resulted in a territorial compromise and an unstable truce.

In the late 1490s, Poland faced pressure from the Crimean Khanate and the Ottoman Empire, while Lithuania faced the Grand Duchy of Moscow. Ivan III of Russia claimed that he inherited rights to all Russian and Orthodox lands after the fall of the Byzantine Empire. His ambitions led to beginnings of the century-long Muscovite–Lithuanian Wars. After the Tatars invaded Volhynia and Podolia in late 1494, John Albert suggested a military and further political alliance to his brother Alexander. He agreed, but negotiations dragged until spring 1498, when the Tatars invaded Podolia and Galicia and took thousands of prisoners. Reacting to these threats and wishing to secure Lithuanian military assistance, Polish nobles agreed with all Lithuanian suggestions and demands.

The Union of Vilnius was based on the Union of Horodło of 1413. It was an alliance of two equal states. It was agreed that future rulers of both countries would be chosen separately, but with consent of the other state. The Union also provided for mutual aid and assistance in various armed conflicts. Historian Tomas Baranauskas interpreted it as the most advantageous for Lithuania of all Polish–Lithuanian unions. However, almost immediately Polish nobles began protesting the union on a technicality – the act referenced the Union of Horodło, which they did not have available.

After the Moldavian loss of Chilia and Cetatea Albă, the Ottoman threat seemed more evident. John I Albert was suzerain of Moldavia, and, when Ștefan asked him for military assistance, they met in 1494 at the conference of Levoča, where together with King Ladislaus II of Hungary and Elector Johann Cicero of Brandenburg, they forged plans for an expedition against the Porte. The objective was to recapture Chilia and Cetatea Albă. It took Poland three years to complete preparations. Their army was made of Polish Crown forces, aided by a number of foreign mercenaries, 400 Teutonic Knights under Grand Master Johann von Tieffen, and a 600 strong unit from Mazovia. Altogether, the Polish army was some 40,000 strong, with 200 cannons.

However, in unexplained circumstances, Ștefan received reports from Hungary that John I Albert prepared to place his own brother, the Polish prince Sigismund (later king, as Sigismund I the Old), on the Moldavian throne.

The campaign started on the wrong foot, with John I Albert entering Moldavia at Hotin and - despite sound advice to the contrary - deciding not to take the fortress, but to go straight for the capital city of Suceava. Later, this would prove a fatal mistake: by this time Stephen's scorched earth tactics were already common knowledge, as he had used it successfully several times before against the Hungarians and the Ottomans; yet John I Albert failed to secure his communication lines with Poland. Supply from home proved impossible and the Poles were forced to live off an already depleted land.

Invasions into Poland and Lithuania from the Crimean Khanate took place in 1502 and 1506 during the reign of King Alexander. In 1506, the Tatars were defeated at the Battle of Kletsk by Michael Glinski.

The Battle of Kletsk was a battle fought on 5 August 1506 near Kletsk (now in Belarus), between the Grand Ducal Lithuanian army, led by Court Marshal of Lithuania Michael Glinski, and the army of the Crimean Khanate, led by Fetih I Giray and Burnaş I Giray, sons of the Khan of Crimea, Meñli I Giray. The battle was one of the first and greatest Lithuanian victories over the Tatars.

The Glinski rebellion was a revolt in 1508 in the Grand Duchy of Lithuania by a group of aristocrats led by Prince Mikhail Glinski in 1508. It grew out of a rivalry between two factions of the nobility during the final years of Grand Duke Alexander Jagiellon. The revolt began when Sigismund I, the new Grand Duke, decided to strip Glinski of his posts based on rumors spread by Jan Zabrzeziński, Glinski's personal enemy. After failing to settle the dispute at the royal court, Glinski and his supporters (mostly relatives) rose up in arms. The rebels swore allegiance to Vasili III of Russia, who was waging war against Lithuania.

The rebels and their Russian supporters failed to achieve military victory. They were allowed to go into exile in Moscow and take their movable property, but their vast land possessions were confiscated.

In the two previous wars, the Moscow state did not succeed in realizing the idea of regaining all the "Kievan inheritance" - the lands of Principality of Smolensk, Principality of Polotsk and Principality of Kiev. The Grand Duchy of Lithuania and Ruthenia did not accept the results of these wars - the loss of some of its eastern lands. At the end of 1512 a new war broke out between the two states. The reason for this was the Lithuanian-Crimean Tatar negotiations and the attack of the Crimean Tatars in May 1512 on the Upper Oka Principalities.

The Moscow government accused the Lithuanians of inciting the Crimean Khanate into Moscow's lands, and in November of that year organized a campaign of troops against Polotsk and Smolensk.

Soon the troops were withdrawn from Polotsk, they remained near Smolensk until March 1513, after which the siege was lifted. In June 1513, Moscow troops launched an offensive in four directions in order to capture Smolensk, Polotsk, Vitebsk and Orsha.

On 8 September, shortly after dawn, Ivan Chelyadnin gave the order to attack. The Muscovite forces attempted to outflank the Lithuanians and Poles by attacking their flanks, which were manned by Polish, Lithuanian light hussar, and Tartar troops. One of the pincers of the attack was commanded by Chelyadnin personally, while the other was led by Prince Bulgakov-Golitsa. The initial attack failed, and the Muscovites withdrew toward their starting positions. Chelyadnin was still confident that the odds, almost 3:1 in his favor, would give him the victory. However, preoccupied with his own wing of the Muscovite forces, he lost track of the other sectors and failed to coordinate a defense against the counterattack by the Lithuanian light and Polish heavy cavalry, which until then had been kept in reserve.

The Lithuanian and Polish light horse and Tartars attacked the overstretched center of the Muscovite lines in an attempt to split them. At the crucial moment the Polish cavalry seemed to waver, then went into retreat. The Muscovites pursued with all their cavalry reserves. The Lithuanian Tartars and Polish cavalry, after retreating for several minutes under chase from the Russians, suddenly turned to the sides. The Muscovite cavalry now found themselves confronted by artillery concealed in the forest. From both sides, Lithuanian forces appeared and proceeded to surround the Muscovites. Ivan Chelyadnin sounded retreat, which soon became somewhat panicked. The Muscovite forces were pursued by the army of the Grand Duchy of Lithuania for five kilometres. The Muscovite defeat is often attributed to repeated failures by Ivan Chelyadnin and Mikhail Golitsa to coordinate their operations.

The First Congress of Vienna was held in 1515, attended by the Holy Roman Emperor, Maximilian I, and the Jagiellonian brothers, Vladislaus II, King of Hungary and King of Bohemia, and Sigismund I, King of Poland and Grand Duke of Lithuania. Previously, Vladislaus and Maximilian had agreed on a Habsburg-Jagiellon mutual-succession treaty in 1506. It became a turning point in the history of Central Europe. After the death of Vladislaus, and later his son and heir, the childless King Louis II at the Battle of Mohács against the Ottomans in 1526, the Habsburg-Jagellion mutual succession treaty ultimately increased the power of the Habsburgs and diminished that of the Jagiellonians.

Polish forces under Grand Crown Hetman Mikołaj Firlej gathered near Koło and in January struck towards Pomesania towards Königsberg, laying siege to Marienwerder (now Kwidzyn) and Preußisch Holland (now Pasłęk). The siege was slow, however, since the Polish forces lacked artillery power. The Polish fleet began a blockade of Teutonic ports. The Knights, in the meantime, took the Warmian city of Braunsberg (now Braniewo). The Polish army received artillery reinforcements in April and took Marienwerder and Preußisch Holland that month, but failed to retake Braunsberg.

The war grew, with Polish forces from the Duchy of Masovia and Gdańsk striking the nearby Teutonic fortifications. Teutonic forces were on defense, waiting for reinforcements from Germany, which arrived in the summer of 1520. In July, the Teutonic army started an offensive, attacking Masovia, Warmia, and Łomża territories, laying siege to Lidzbark Warmiński. In August, another group of German reinforcements attacked Greater Poland, taking Międzyrzecz. The Germans took Wałcz, Chojnice, Starogard Gdański, and Tczew and started a siege of Gdańsk, but they retreated when faced with Polish reinforcements and plagued by financial troubles (German reinforcements, mostly mercenaries, refused to fight until paid). Polish forces retook Tczew, Starogard, and Chojnice. The Teutonic Knights retreated towards Oliwa and Puck, pursued by Polish forces. The Polish side was then struck with financial troubles, and the "pospolite ruszenie" forces were also tired. The Teutonic Knights seized their chance and launched a counteroffensive, taking Nowe Miasto Lubawskie and approaching Płock and Olsztyn. Olsztyn was successfully defended by the Poles under the command of Nicolaus Copernicus.

At that point, the Ottoman Empire invaded Hungary, and the new Holy Roman Emperor, Charles V, demanded that the Teutonic Knights and Poles stop their hostilities and aid the defense of Europe against the infidels. Both sides, tired with the war, agreed to an armistice on 5 April 1521 in the Compromise of Thorn.

The teachings of Martin Luther were accepted most readily in the regions with strong German connections: Silesia, Greater Poland, Pomerania and Prussia. In Danzig (Gdańsk) in 1525 a lower-class Lutheran social uprising took place and was bloodily subdued by Sigismund I; after the reckoning he established a representation for the plebeian interests as a segment of the city government. Königsberg and the Duchy of Prussia under Albrecht Hohenzollern became a strong center of Protestant propaganda dissemination affecting all of northern Poland and Lithuania. Sigismund quickly reacted against the "religious novelties", issuing his first related edict in 1520, banning any promotion of the Lutheran ideology, or even foreign trips to the Lutheran centers. Such attempted (poorly enforced) prohibitions continued until 1543.

The Prussian Homage or Prussian Tribute was the formal investment of Albert of Prussia as duke of the Polish fief of Ducal Prussia. In the aftermath of the armistice ending the Polish-Teutonic War Albert, Grand Master of the Teutonic Knights and a member of the House of Hohenzollern, visited Martin Luther at Wittenberg and soon thereafter became sympathetic to Protestantism. On 10 April 1525, two days after signing of the Treaty of Kraków which officially ended the Polish–Teutonic War (1519–21), in the main square of the Polish capital Kraków, Albert resigned his position as Grand Master of the Teutonic Knights and received the title "Duke of Prussia" from King Zygmunt I the Old of Poland. In the deal, partially brokered by Luther, the Duchy of Prussia became the first Protestant state, anticipating the Peace of Augsburg of 1555. The investiture of a Protestant fief of Duchy of Prussia was better for Poland for strategic reasons than a Catholic fief of State of Teutonic Order in Prussia, formally subject to the Holy Roman Emperor and the Papacy.

As a symbol of vassalage, Albert received a standard with the Prussian coat of arms from the Polish king. The black Prussian eagle on the flag was augmented with a letter "S" (for Sigismundus) and had a crown placed around its neck as a symbol of submission to Poland.

Chicken War or Hen War is the colloquial name for a 1537 anti-royalist and anti-absolutist rokosz (rebellion) by the Polish nobility. The derisive name was coined by the magnates, who for the most part supported the King and claimed that the conflict's only effect was the near-extinction of the local chickens, eaten by the nobles gathered for the rokosz at Lwów, in Ruthenian Voivodeship. The magnates' choice of "kokosz"—meaning "an egg laying hen"—may have been inspired by a play on words between "kokosz" and the similar-sounding "rokosz". The Chicken War was the first rokosz of the Szlachta in Polish history.

Sigismund II Augustus was the only son of Italian-born Bona Sforza and Sigismund the Old. From the beginning he was groomed and extensively educated as a successor. In 1529 he was crowned vivente rege while his father was still alive. Sigismund Augustus continued a tolerance policy towards minorities and maintained peaceful relations with neighbouring countries, with the exception of the Northern Seven Years' War which aimed to secure Baltic trade. Under his patronage, culture flourished in Poland; he was a collector of tapestries from the Low Countries and collected military memorabilia as well as swords, armours and jewellery. Sigismund Augustus' rule is widely considered as the apex of the Polish Golden Age; he established the first regular Polish navy and the first regular postal service in Poland, known today as Poczta Polska. In 1569 he oversaw the signing of the Union of Lublin between Poland and the Grand Duchy of Lithuania, which formed the Polish–Lithuanian Commonwealth and introduced an elective monarchy.

Sigismund Augustus was the last male member of the Jagiellons. Following the death of his sister Anna in 1596 the Jagiellonian dynasty came to an end.

The Polish Golden Age was the Renaissance period in Poland and the Grand Duchy of Lithuania, roughly corresponding to the period of rule of the King Sigismund I the Old and his son, Sigismund II Augustus, the last of the Jagiellonian Dynasty monarchs, until his death in 1572.

In the 16th century the Commonwealth grew to almost 1 million km2, with a population of 11 million. It prospered from its enormous grain, wood, salt, and cloth exports to Western Europe via the Baltic Sea ports of Gdańsk, Elbląg, Riga, Memel, and Königsberg. The Commonwealth's major cities included Poznań, Kraków, Warsaw, Lviv, Vilnius, Toruń, and, for a time in the 17th century, Kiev and Smolensk. The Commonwealth army was able to defend the realm from foreign invasion, and also participated in aggressive campaigns against Poland's neighbors. As Polonization followed in conquered territories, at least among the politically influential classes, the Polish language became the lingua franca of Central and Eastern Europe.

During its Golden Age, the Commonwealth was regarded as one of the most powerful states in Europe. It had a unique system of government, known as Golden Liberty, in which all the nobility (szlachta), regardless of economic status, were considered equal and enjoyed extensive legal rights and privileges.

The Livonian War (1558–1583) was fought for control of Old Livonia (in the territory of present-day Estonia and Latvia), when the Tsardom of Russia faced a varying coalition of the Dano-Norwegian Realm, the Kingdom of Sweden, and the Union (later Commonwealth) of the Grand Duchy of Lithuania and the Kingdom of Poland.

During the period 1558–1578, Russia dominated the region with early military successes at Dorpat (Tartu) and Narva.

In 1576, Stephen Báthory became King of Poland as well as Grand Duke of Lithuania and turned the tide of the war with his successes between 1578 and 1581, including the joint Swedish–Polish–Lithuanian offensive at the Battle of Wenden. This was followed by an extended campaign through Russia culminating in the long and difficult siege of Pskov. Under the 1582 Truce of Jam Zapolski, which ended the war between Russia and Poland–Lithuania, Russia lost all its former holdings in Livonia and Polotsk to Poland–Lithuania. The following year, Sweden and Russia signed the Truce of Plussa with Sweden gaining most of Ingria and northern Livonia while retaining the Duchy of Estonia.

The Northern Seven Years' War (also known as the Nordic Seven Years' War, the First Northern War or the Seven Years War in Scandinavia) was fought between the Kingdom of Sweden and a coalition of Denmark–Norway, Lübeck, and Poland–Lithuania between 1563 and 1570. The war was motivated by the dissatisfaction of King Frederick II of Denmark with the dissolution of the Kalmar Union, and the will of King Eric XIV of Sweden to break Denmark's dominating position. The fighting continued until both armies had been exhausted, and many men died. The resulting Treaty of Stettin was a stalemate, with neither party gaining any new territory.

The Union of Lublin was signed on 1 July 1569 in Lublin, Poland, and created a single state, the Polish–Lithuanian Commonwealth, one of the largest countries in Europe at the time. It replaced the personal union of the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania with a real union and an elective monarchy, since Sigismund II Augustus, the last of the Jagiellons, remained childless after three marriages. In addition, the autonomy of Royal Prussia was largely abandoned. The Duchy of Livonia, tied to Lithuania in real union since the Union of Grodno (1566), became a Polish–Lithuanian condominium.The Commonwealth was ruled by a single elected monarch who carried out the duties of King of Poland and Grand Duke of Lithuania, and governed with a common Senate and parliament (the Sejm). The Union is seen by some as an evolutionary stage in the Polish–Lithuanian alliance and personal union, necessitated also by Lithuania's dangerous position in wars with Russia. == History.

The Sandomierz Agreement was an agreement reached in 1570 in Sandomierz between a number of Protestant groups in the Polish–Lithuanian Commonwealth. It was intended to unite different creeds of the Protestant Reformation, such as the Calvinists, the Lutherans, and the Bohemian Brethren, and to face Counter-Reformation as a united front. The Polish Brethren did not participate in the talks that resulted in the agreement, signed on April 14, 1570. Signatories of the consensus agreed to respect each other's preachers and sacraments. Furthermore, united synods were planned. The idea of a parliament bill was raised, in which Protestants were to be treated on equal terms with Catholics.