Final answer:
The US and the USSR signed the Anti-Ballistic Missile Treaty in May 1972, which was part of the larger Strategic Arms Limitation Talks leading to a reduction of anti-ballistic missile sites to two initially, and later to one per country.
Step-by-step explanation:
The Anti-Ballistic Missile Treaty, a pivotal component of the Strategic Arms Limitation Treaty (SALT) negotiations, was signed by the United States and the Soviet Union in May 1972 during President Nixon's visit to Moscow. Reflecting a diplomatic move towards détente, the treaty limited both nations to deploying just two anti-ballistic missile systems. A subsequent protocol in 1974 further restricted this to a single site for each country.
These agreements aimed at curbing the proliferation of anti-ballistic missile systems were integral to broader efforts in reducing the risk of nuclear conflict. In addition to arms control, the agreements fostered cooperation in scientific and technological exchanges, symbolizing a thaw in Cold War tensions and even included a commitment to a joint space mission, marking a significant chapter in U.S.-Soviet relations during the era of détente.