Which treaty limited the U.S. and the USSR to two Anti-ballistic Missile sites?

The Anti-Ballistic Missile Treaty is significant in the context of Cold War arms control as it explicitly limited the number of anti-ballistic missile (ABM) systems that the U.S. and the USSR could possess. Signed in 1972, this treaty allowed both countries to maintain only two ABM sites each, thereby restricting their ability to develop extensive missile defense systems. The primary goal was to enhance stability by ensuring that both nations would remain vulnerable to each other's offensive nuclear capabilities, which would theoretically encourage diplomatic resolution over military confrontation.

This treaty marked a critical step in limiting the arms race and fostering a climate of cooperation, recognizing that an extensive deployment of ABM systems could lead to new arms buildups and undermine mutual deterrence. The other treaties mentioned serve different purposes, such as preventing nuclear proliferation or reducing strategic nuclear arsenals, but do not address limitations specifically concerning anti-ballistic missile systems like the Anti-Ballistic Missile Treaty does.