Estimates for the total number of casualties in the war vary, because many deaths went unrecorded. Most suggest that some 60 million people died in the war, including about 20 million military personnel and 40 million civilians.Many of the civilians died because of deliberate genocide, massacres, mass-bombings, disease, and starvation. The Soviet Union lost around 27 million people during the war, including 8.7 million military and 19 million civilian deaths. A quarter of the people in the Soviet Union were wounded or killed. Germany sustained 5.3 million military losses, mostly on the Eastern Front and during the final battles in Germany. Of the total number of deaths in World War II, approximately 85 per cent—mostly Soviet and Chinese—were on the Allied side. Many of these deaths were caused by war crimes committed by German and Japanese forces in occupied territories. An estimated 11 to 17 million civilians died as a direct or as an indirect result of Nazi racist policies, including the Holocaust of around 6 million Jews, half of whom were Polish citizens, along with at least 1.9 million ethnic Poles. Millions of other Slavs , Roma, homosexuals, and other ethnic and minority groups were also killed. Between 1941 and 1945, over 200,000 ethnic Serbs, along with gypsies and Jews, were persecuted and murdered by the Axis-aligned Croatian Ustaše in Yugoslavia. Also, over 100,000 Poles were massacred by the Ukrainian Insurgent Army in the Volhynia massacres, between 1943 and 1945.

Over how many years were 100,000 Poles massacred by the Axis-aligned Croatian Ustaše in Yugoslavia?
2