World War II , or the Second World War ( also written in WWII or WW2 ) , was a war fought on almost every continent on the world from 1939 and 1945 . Most of the world 's countries fought in the war , including all of the great powers , forming into two military alliances : the Allies and the Axis . World War II , or the Second World War ( often abbreviated as WWII or WW2 ) , was a global military conflict lasting from 1939 to 1945 , which involved most of the world 's nations , including all of the great powers : eventually forming two opposing military alliances , the Allies and the Axis .