United States, Russia, & Great Britain!
The United States tried to remain neutral during WW2 but Axis Powers bombed the Pearl Harbor forcing the U.S. to join the war with no other choice. The Allied Powers formed mostly as a defensive against the Axis. After Germany invading Poland Great Britain declared war on them and U.S. joined because of the bombing.
After WWII United States and Russia became competitors causing the COLD WAR. After the war even though they won a consequence was still issued called the United Nations. This stated that there would be no more wars prior to WWII.