So the Air Force didn't come into existence until the late 1940's.  But the advent of air power really changed the whole way war was fought.  It added an additional level to the playing field. 

Air power is more effective offensively.  You can strike several places at once, you can drop create a lot of damage at once, you can even be so precise as to just destroy part of a building.  It's much harder to guard airspace than to invade it.  Enemy aircraft can come at you from any direction.  Enemy aircraft can maneuver to avoid detection. 

In some ways, air power can be seen as skipping past the middlemen of land and sea power.  While a force may be blockading ships or trying to march by land through obstacles, a plane can fly 35,000 feet above it and bypass everything completely. 

Alright, now I'm sick of the subject.