Citation for you from MS Encarta, article “Poland” 

There is nothing like supposition about history of the “Recovered territories”. If you prefer you can check it in other encyclopaedia like “Britannica” or some other – neutral of course ) Good lack with studies.

The Piast Dynasty  Mieszko converted the Poles to Christianity in order to compete better with the crusading and marauding Germans. During the reign (992-1025) of his son, Boleslaw I, the Christian church was firmly established in Poland. Boleslaw also conducted successful wars against Holy Roman Emperor Henry II and considerably expanded the Polish domain. He was crowned king by the pope in 1025. At his death, Poland extended beyond the Karpaty Mountains (Carpathian Mountains) and the Odra and Dniester rivers.