In the military sphere, World War II marked the coming of age of airpower. Advanced aircraft and guided missiles (developed late in the war) made the battleship, once the queen of the world's oceans, and fixed fortifications such as coastal artillery obsolete. While the pendulum continues to swing in this never-ending competition, air powers are now a full partner in any military action.
The war was the high-water mark for mass armies. While huge conscript armies were seen again (during the Korean War and in several African conflicts), after this victory the major powers relied upon small highly-trained and well-equipped militaries.
Perhaps most important of all, World War II ushered in the nuclear era, with the dropping of the first atomic bombs on the Japanese of Hiroshima and Nagasaki in 1945.