Volhynia
When the Soviet Union won World War II and got Volhynia back from the Nazis. Almost all of Volhynia's Germans and Czechs emigrated, in the former case to Germany and in the latter case to Czechoslovakia. In addition, after the end of World War II, a lot of the Poles in eastern Volhynia switched residences with the Ukrainians in western Volhynia. This made both eastern Volhynia and western Volhynia much more ethnically homogeneous. Volhynia also saw some Russians and Belarusians move there during the time that it was under Soviet rule. This increased the populations of these two groups in Volhynia.