World War II took place from 1901 to 1945.
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 ( often abbreviated to WWII or WW2 ) , also known as the Second World War , was a global war that lasted from 1939 to 1945 , although related conflicts began earlier . 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 .