The Anti Ballistic Missile Treaty sought to cement the system of mutually assured destruction as the lynchpin of strategic balance. The ABM treaty restricted the ability of the two superpowers to defend themselves from nuclear strike. In theory, this made a first strike to prevent the introduction of destabilizing defensive systems unnecessary. Both nations were allowed to defend either their capital or one field of ICBMs with a missile defense system. The Soviets deployed such a system around Moscow. Ultimately, the US abandoned its system deployed in Grand Forks, North Dakota.
Time: Mid War
Removed after event: No
Its most common use is as a free 4 Ops coup after DEFCON drops to 2. (Somewhat ironic that an anti-ballistic missile treaty is intended to launch coups, but that’s Cold War logic for you I suppose.) This is a pretty self-explanatory way to dramatically alter the dynamics of South America or Central America, and more rarely a way to really, really lock up an African battleground. For the US, they get a small bonus here with NORAD.
You can also headline this to conduct operations in the headline. Headlining it (or playing it on AR1, as USSR) gives you the chance to perform the rare Asia coup, possibly flipping Thailand or Pakistan.
Of course, being able to conduct operations in the headline is an all-around useful tactic. The US can perform the SALT-ABM trick: use SALT Negotiations to reclaim ABM Treaty to your hand at the end of the turn, pushing DEFCON up two levels. Then as DEFCON rises to 5 next turn, you headline ABM Treaty and get to conduct operations in Europe in the headline phase: typically a series of realignments, though occasionally you might see an Italy coup. The USSR can do this too, but it’s a little bit trickier: first, you’re giving the US a free (albeit -1) battleground coup on AR7; second, the US might headline Defectors; third, a US 4 Ops headline would trigger before yours. The US faces none of these problems.