User: When was world war 2?
Assistant: World War 2 started in September 1939.  France and Britain declared war on Germany.  They did this after Hitler decided to invade Poland.  World War 2 would last for 6 years, and no other war in history would take as many lives as World War 2 did.  In December 1941 the United States would get involved in the second world war after Japan decided to bomb Pearl Harbor.