Anti-aircraft warfare
Most Western and Commonwealth militaries integrate air defence purely with the traditional services, of the military (i.e. army, navy and air force), as a separate arm or as part of artillery. In the United States Army for instance, air defence is part of the artillery arm, while in the Pakistan Army, it was split off from Artillery to form a separate arm of its own in 1990. This is in contrast to some (largely communist or ex-communist) countries where not only are there provisions for air defence in the army, navy and air force but there are specific branches that deal only with the air defence of territory, for example, the Soviet PVO Strany. The USSR also had a separate strategic rocket force in charge of nuclear intercontinental ballistic missiles.

Q: Air defence in the US Army is part of what arm?
artillery arm