1939–1945 global conflict
Source (Wikipedia): World War II

World War II or the Second World War was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all the world’s countries—including all the great powers—participated, with many investing all available economic, industrial, and scientific capabilities in pursuit of total war, blurring the distinction between military and civilian resources. Tanks and aircraft played major roles, with the latter enabling the strategic bombing of population centres and delivery of the only two nuclear weapons ever used in war.