Mieszko I - the beginning of the Polish state.

Mieszko I is the ruler of the Piast dynasty (probably the fourth in turn, według Galla Anonima przed Mieszkiem było jeszcze 3 Piastów :

1. Siemowit (grand grandfather) 2. Leszek (grandfather) 3. Siemomysł (father)).

He was born around 935, its reign began around the year 960.
In 963 and 965, Mieszko's dependence on the German king was established.
In 965 the Polish prince made an alliance with the Czech prince - Boleslaw I Srogi,
which was strengthened by the marriage of Mieszko and Dobrawa.

But

In 966 something that is considered to be the beginning of the Polish state. Baptism of Poland.
In 968 the Christianization of Poland began. The missionary bishopric was established in Poznań.

,,Mieszko I" J. Matejki

Alliance with Czechs did not last forever. When Dobrawa died, Mieszko took Silesia belonging to the Czech Republic.
The conflict began when Boleslaw II demanded to cast Silesia probably.

Between 990 and 991, Mieszko and his whole family gave up their country to the care of the Holy See.

Sorry for my bad english, England isn't my city :P.