On September 1, 1939, World War II started when Nazi Germany attacked Poland. The Soviet Union attacked Poland on September 17, 1939. Warsaw was defeated on September 28, 1939. Poland was split into two pieces, one half owned by Nazi Germany, the other by the Soviet Union. More than 6 million Polish people died, and half of these people were Jewish. Most of these deaths were part of the Holocaust, in which 6 million Jews were killed. At the war's end, Poland's borders were moved west, pushing the eastern border to the Curzon line. The western border was moved to the Oder-Neisse line. The new Poland became 20% smaller by 77,500 square kilometers (29,900 sq mi). The shift forced millions of Poles, Germans, Ukrainians, and Jews to move.