United States Air Force

Multiple realities
(covers information from several alternate timelines)

The United States Air Force (USAF) was a branch of the United States military on Earth during the 20th century. Its predecessor from the 1940s was the United States Army Air Forces. (DS9: "Little Green Men") The Air Force was chiefly responsible for deploying military aircraft, although the Navy operated aircraft carriers. The Air Police was a branch responsible for defending airbases and facilities. (TOS: "Tomorrow is Yesterday")

The Air Force also maintained a fleet of intercontinental ballistic missiles, as part of the Earth Cold War and later World War III. Zefram Cochrane eventually converted one such Titan II missile into the prototype warp ship Phoenix. (Star Trek: First Contact)

Markings in the Bozeman missile complex from which Zefram Cochrane launched the Phoenix suggest that it was previously operated by the USAF.

In 1953, Roy Ritterhouse drew an illustration for Incredible Tales magazine, depicting a space station that was operated by the US Air Force. The illustration later became part of the "Deep Space Nine" novella. (DS9: "Far Beyond the Stars")

In 1969, the Air Force detected the USS Enterprise, after it had arrived through an accidental time warp from 2267. Alarmed, the Strategic Air Command informed the Air Defense Command, which scrambled an F-104 Starfighter. Its pilot, Captain John Christopher, attempted to intercept the Enterprise. In one version of history, his plane was destroyed and he was taken aboard the ship. In the historical timeline "recreated" by Spock's slingshot method of returning to the future, the ship vanished before Christopher could reach it, and he returned to his base, unaware of what he had seen. (TOS: "Tomorrow is Yesterday")

When the USS Voyager was accidentally transported to 1996, and flew through Earth's atmosphere over Los Angeles, the United States military took the incident quite seriously. Captain Janeway feared that if the ship risked another pass through the lower atmosphere, it risked being intercepted by the Air Force. (VOY: "Future's End, Part II")