According to Wikipedia, on May 26, 1972, the United States and Soviet Union signed the anti-ballistic missle (ABM) treaty. The treaty was signed by President Nixon and General Secretary of the Central Committee of the Communist Party of the Soviet Union, Leonid Brezhnev.
The treaty was intended to limit arms proliferation, after advancements in long-range missles such as the MIRV brought the Soviet Union into parity with the United States. The financial cost of an ABM, which could take down only one warhead per ABM, compared to a multi-warhead missle like the MIRV would have been staggering. The treaty was intended to limit both offensive systems as well as defensive systems.
Though ABM treaties had been proposed as early as 1967, but earlier proposals were rejected. The ABM treaty was signed in Moscow, and then ratified by the senate in August 1972.