Complex sentence: At the onset of World War II, on 1 September 1939 The Nazi Germany Invades Poland by Treaty Versaillies., but was overwhelmed by the German attack in September 1939, which was followed on 17 September 1939 by an invasion by the Soviet Union.
Split into simpler sentences: At the onset of World War II, on 1 September 1939 Nazi Germany invaded Poland. Polish forces were overwhelmed by the German attack in September 1939, which was followed on 17 September 1939 by an invasion by the Soviet Union.