World War II was fought between the Axis powers and the Allies of World War II.
The Allies of World War II , called the United Nations from the 1 January 1942 declaration , were the countries that together opposed the Axis powers during the Second World War ( 1939 -- 1945 ) . World War II . It involved the vast majority of the world 's countries -- including all of the great powers -- eventually forming two opposing military alliances : the Allies and the Axis .