The main opponents in world war 2 were the

The Axis Powers (Germany, Italy, Japan) fought the Allies (France, Great Britain, the United States, the Soviet Union, China).

It started when Germany invaded Poland on September 1, 1939. That's when Great Britain and France declared war on Germany on September 3.

America didn't enter WW2 until Japanese bombed the American fleet in Pearl Harbor, Hawaii, on December 7, 1941.

The allies and the axis