Aerial Warfare first began in the First World War when recon pilots began using various offensive tactics against each other. Bricks, darts and even handheld bombs were used as weapons. Eventually planes were fitted with standard issue machine-guns and over the years, dogfighting became a staple part of war.
In Rise of Nations, various aircraft are available at the airfield.
Aircraft enjoy the benefit of flying over terrain, that ground forces may be forced to negotiate. They have better speed and manuverability, making them great scouts, or patrols.
However most aircraft are lightly armored, and fixed-wing aircraft (Bombers and Fighters), have a limited fuel supply; Once diminished, they automatically land at the nearest vacant Airbase to refuel before recommencing their mission; sea-borne aircraft return to their designated aircraft carrier to rearm instead. Helicopters do not need to refuel however.
Should all friendly airbases, and aircraft carriers, are destroyed all airborne fixed-wing aircraft will crash and you will be notified to build a new airfield/carrier in order to allow them to relocate and refuel.
Even though some ground forces can fire upon aircraft. More effective means of surface-to-air defense are available; but they should be protected from enemies since they cannot engage ground targets.