The establishment and expansion of the Polish state in the 10th century can be traced back to the Polans, a West Slavic tribe that settled in the Greater Poland region, utilizing the strategic locations of Giecz, Poznań, Gniezno, and Ostrów Lednicki. During the early 10th century, significant fortification and territorial expansion began, particularly around 920-950. This period set the stage for the evolution of these tribal lands into a more centralized state under the leadership of the Piast dynasty, notably Mieszko I.

Mieszko I, first mentioned in contemporary sources by Widukind of Corvey in the mid-960s, significantly shaped the early Polish state. His rule saw both military confrontations and strategic alliances, such as his marriage in 965 to Doubravka, a Christian Bohemian princess, which precipitated his conversion to Christianity on April 14, 966. This event, known as the Baptism of Poland, is considered foundational to the Polish state. Mieszko's reign also marked the beginning of Poland's expansion into territories like Lesser Poland, the Vistulan lands, and Silesia, which were integral in forming a territory approximating modern-day Poland.

The Polans, under Mieszko’s rule, began as a tribal federation and evolved into a centralized state that merged with other Slavic tribes. By the late 10th century, Mieszko’s realm covered an area of about 250,000 km² and housed just under one million people. The political landscape of Mieszko's Poland was complex, characterized by both alliances and rivalries within the region. His diplomatic relations with the Holy Roman Empire, through alliances and tributes, were particularly significant.

Mieszko’s military engagements with neighboring tribes and states, such as the Velunzani, Polabian Slavs, and the Czechs, were pivotal in securing and expanding Polish territories. The Battle of Cedynia in 972 against Margrave Odo I of the Saxon Eastern March was a notable victory that helped consolidate Mieszko's control over Pomeranian territories up to the Oder River.

By the end of his reign around 990, Mieszko had established Poland as a major power in central-eastern Europe, culminating in his submission of the country to the authority of the Holy See through the Dagome iudex document. This act not only solidified the Christian character of the state but also placed Poland firmly within the broader European political and religious landscape.