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lazy wikipedia copypaste from https://en.wikipedia.org/wiki/Recovered_Territories because I didn't bother looking through legitimate sources
People from all over Poland quickly moved in to replace the former German population in a process parallel to the expulsions, with the first settlers arriving in March 1945.[60] These settlers took over farms and villages close to the pre-war frontier while the Red Army was still advancing.[60] In addition to the settlers, other Poles went for "szaber" or looting expeditions, soon affecting all former eastern territories of Germany.[60] On 30 March 1945, the Gdansk Voivodeship was established as the first administrative Polish unit in the "recovered" territories.[61] While the Germans were interned and expelled, close to 5 million settlers[62][63] were either attracted or forced to settle the areas between 1945 and 1950. An additional 1,104,000 people had declared Polish nationality and were allowed to stay (851,000 of those in Upper Silesia), bringing up the number of Poles to 5,894,600 as of 1950.[57] The settlers can be grouped according to their background:
•settlers from Central Poland moving voluntarily (the majority)[62]
•Poles that had been freed from forced labor in Nazi Germany (up to two million)[62][64]
•so-called "repatriants": Poles expelled from the areas east of the new Polish-Soviet border were preferably settled in the new western territories, where they made up 26% of the population (up to two million)[62][64]
•non-Poles forcibly resettled during the Operation Vistula in 1947. Large numbers of Ukrainians were forced to move from south-eastern Poland under a 1947 Polish government operation aimed at dispersing, and therefore assimilating, those Ukrainians who had not been expelled eastward already, throughout the newly acquired territories. Belarusians living around the area around Białystok were also pressured into relocating to the formerly German areas for the same reasons. This scattering of members of non-Polish ethnic groups throughout the country was an attempt by the Polish authorities to dissolve the unique ethnic identity of groups like the Ukrainians, Belarusians and Lemkos,[65] and broke the proximity and communication necessary for strong communities to form.