The first known states to hold power there were those of Greater Moravia at end of the 9th century and Bohemia early in the 10th century.
Wikipedia forgot here about the tribal state of Vistulans, who were local inhabitants in that region. If such a state existed (most likely it did):
http://en.wikipedia.org/wiki/Vistulans
But indeed, Great Moravia controlled that area. Later it was independent for some time. Then came Bohemia. And only then Poland.
In 1335, in the Treaty of Trentschin, Casimir relinquished "in perpetuity" his claims to Silesia.
To be precise, in that Treaty, Casimir relinquished his claims not to entire Silesia, but to the following Piast dukes and duchies (listed in the treaty):
- Land of Wrocław
- Land of Głogów
- Bolesław, duke of Legnica-Brzeg
- Henryk, duke of Żagań-Krosno
- Konrad, duke of Oleśnica
- Jan, duke of Ścinawa
- Bolko, duke of Opole
- Bolko, duke of Niemodlin
- Albert, duke of Strzelce
- Władysław, duke of Koźle-Cieszyn
Lands and duchies listed above comprised majority of Silesia - not entire region, though.
But Casimir soon changed his mind (as soon as he signed peace with Teutonic Order in 1343), and another Polish-Czech War for Silesia started in 1345.
It ended in 1348 with the treaty of Namysłów:
http://en.wikipedia.org/wiki/Treaty_of_Namslau
In the treaty of Namslau in 1348 Casimir again renounced his claims to most of Silesia (excluding still independent duchies and those parts which were already his), but this time he did not relinquish them "in perpetuity" - i.e. this time he didn't guarantee that he was not going to change his mind.
And there was also the treaty of Prague in 1356, which was an alliance between Casimir and Charles IV, as well as exchange of territories:
http://pl.wikipedia.org/wiki/Układ_praski
http://translate.google.com/translate?sl=pl&tl=en&u=http://pl.wikipedia.org/wiki/Uk%C5%82ad_praski
In the 15th century several changes were made to Silesia's borders. Parts of the territories which had been transferred to the Silesian Piasts in 1178 were bought by the Polish kings in the second half of the 15th century (the Duchy of Oświęcim in 1457; the Duchy of Zator in 1494).
Not all parts of Silesia reunited into Poland are listed here. There were also:
- Duchy of Namysłów-Kluczbork-Byczyna (reunited in 1341 by King Casimir) ---> lost again in 1356
- Land of Wschowa (reunited in 1343 by King Casimir) ---> remained in Poland
- Duchy of Siewierz (reunited in 1443) ---> remained in Poland
- Duchy of Oświęcim (reunited in 1457) ---> remained in Poland
- Duchy of Zator (reunited in 1494) ---> remained in Poland
- Duchy of Opole (acquired in 1645 by King Władysław IV) ---> lost in 1666
- Duchy of Racibórz (acquired in 1645 by King Władysław IV) ---> lost in 1666
The Germanization has started already during the reign of Premysl Otokar I
OK but German immigration is not the same as Germanization.
Unless you want to tell me that there is currently Turkization of Germany.
German immigration into Bohemia and the growth of towns in what had until that point been forest lands.
And to Poland first German immigrants came during the early 1200s. In Poland initially they also settled mostly in sparsely inhabited lands - for example in Silesia first German immigrants colonized the Sudetes (south-western part of Silesia), which before their arrival had been sparsely populated and forested.