Nazists with Adolf Hitler attacked Poland on the 1st of September 1939.
Because of that sneak attack 3rd of September Great Britain & France declared war on Germany. On the 17th of September according to agreement between Molotow and Ribbentrop, the Soviet Union attacked Poland from the other side. Hitler and Stalin diveded Poland between them in peace, but this fragile pact had no future, each side knew about that. The people responsible for IInd WW are obviously Hitler and Stalin, and their nations, as well. War is not an act of units, but entire nations.