In the 1970s, the Soviet Union achieved rough nuclear parity with the United States, and surpassed it by the end of that decade with the deployment of the SS-18 missile. It perceived its own involvement as essential to the solution of any major international problem. Meanwhile, the Cold War gave way to "Détente" and a more complicated pattern of international relations in which the world was no longer clearly split into two clearly opposed blocs. Less powerful countries had more room to assert their independence, and the two superpowers were partially able to recognize their common interest in trying to check the further spread and proliferation of nuclear weapons ("see" SALT I, SALT II, Anti-Ballistic Missile Treaty).