Allies vs Axis, started mostly by fascist imperialism on the part of the Axis powers. During the war, Germany at its peak had taken over most of Europe, Japan had taken most of the Pacific island nations, and Italy had taken most of northern Africa. Ended in 1945. Perhaps the most significant event of the 20th century, and this dictionary can't possibly begin to cover all of it. See also: every show on the History Channel.
Thirty years from now when you're sitting around your fireside with your grandson on your knee, and he asks you, "What did you do in the great World War Two?" -- you won't have to say, "Well, I shoveled shit in Louisiana."
-- General George Patton