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.