The forced relocation of ethnic Germans from Eastern Europe was a sore point for the Americans and British. Even in parts of pre-War Ukraine and Poland there were German minories that had lived there for centuries.
None the less, they agreed to territorial boundaries being shifted, however, Stalin was the one who was adamant about having every German west of the Oder-Nesse. As possession is also nine tenths of the law, Atlee and Truman relented.
The result? Russofication of the Baltic States, Kaliningrad, Ukraine, and the expansion of the Soviet sphere of influence that eventually led to the Cold War.
That decision has now come to rear its head 80 years after the fact with Putin believing the 2 generations of transplanted Russians have been there for millenia and thus need saving and protecting.