The period of rule by the Piast dynasty between the 10th and 14th centuries is the first major stage of the history of the Polish state. The dynasty was founded by a series of dukes listed by the chronicler Gall Anonymous in the early 12th century: Siemowit, Lestek and Siemomysł. It was Mieszko I, the son of Siemomysł, who is now considered the proper founder of the Polish state at about 960 AD. The ruling house then remained in power in the Polish lands until 1370. Mieszko converted to Christianity of the Western Latin Church in an event known as the Baptism of Poland in 966, which established a major cultural boundary in Europe based on religion. He also completed a unification of the Lechitic tribal lands that was fundamental to the existence of the new country of Poland.
Following the emergence of the Polish state, a series of rulers converted the population to Christianity, created a kingdom of Poland in 1025 and integrated Poland into the prevailing culture of Europe. Mieszko's son Bolesław I the Brave established a Roman Catholic Archdiocese in Gniezno, pursued territorial conquests and was officially crowned in 1025 as the first king of Poland. The first Piast monarchy collapsed with the death of Mieszko II Lambert in 1034, followed by its restoration under Casimir I in 1042. In the process, the royal dignity for Polish rulers was forfeited, and the state reverted to the status of a duchy. Duke Casimir's son Bolesław II the Bold revived the military assertiveness of Bolesław I, but became fatally involved in a conflict with Bishop Stanislaus of Szczepanów and was expelled from the country.
Bolesław III, the last duke of the early period, succeeded in defending his country and recovering territories previously lost. Upon his death in 1138, Poland was divided among his sons. The resulting internal fragmentation eroded the initial Piast monarchical structure in the 12th and 13th centuries and caused fundamental and lasting changes.
Konrad I of Masovia invited the Teutonic Knights to help him fight the Baltic Prussian pagans, which led to centuries of Poland's warfare with the Knights and the German Prussian state.
In 1320, the kingdom was restored under Władysław I the Elbow-high, then strengthened and expanded by his son Casimir III the Great. The western provinces of Silesia and Pomerania were lost after the fragmentation, and Poland began expanding to the east. The period ended with the reigns of two members of the Capetian House of Anjou between 1370 and 1384. The consolidation in the 14th century laid the base for the new powerful kingdom of Poland that was to follow.
10th–12th century
Mieszko I and the adoption of Christianity in Poland (ca. 960–992)
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 these 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 in area, with an approximate population of under one million.