Looking for something specific? I mean, a comprehensive answer would have to start at the Teutonic Knights. The Hanseatic League and German nobility's relationship with countries like Russia fostered Ostsiedlung. Socioeconomic and cultural changes brought the manorial system where Germans enjoyed influence disproportionate to their actual numbers. They played a huge role in the region's history. Hitler wanted to use these historical ties to lay claim to the region until they agreed to Soviet occupation and Germans were ethnically cleansed. I'm happy to talk about any or all of these things if you want.