Following the German invasion of Poland in 1939, masses of Polish citizens served in the Wehrmacht and the Waffen-SS, in particular citizens from parts of Poland annexed to Germany such as Upper Silesia and Pomerania. Service in the German military was universal in nature in these areas, however assessing the number of ethnic Poles involved is difficult due to the fluidity of national identity. At the low end, Polish estimates often place the number of native Poles at 250,000.