New START is the latest in a series of agreements between the United States and Russia — or its predecessor, the Soviet Union — to reduce the size of their respective nuclear arsenals. The two superpowers raced to build their nuclear weapons stockpiles in the early decades of the Cold War, in a risky competition that raised fears of a mutually destructive nuclear war. In the late 1960s, President Lyndon B. Johnson called for talks with Moscow to limit each side’s strategic arms. Formal Strategic Arms Limitation Talks (SALT) began under Johnson’s successor, President Richard M. Nixon, in Finland in 1969. In 1972, Nixon and Soviet General Secretary Leonid Brezhnev signed a treaty on Anti-Ballistic Missile defense systems — meant to shoot down incoming missiles — and an interim agreement placing restrictions on the construction of intercontinental ballistic missile (ICBM) silos. Under subsequent agreements, including START I and SORT, both countries pared down their nuclear arsenals substantially. But arms control negotiations were often rocky, and other proposed treaties never entered into force. In 2002, the George W. Bush administration unilaterally withdrew from the Anti-Ballistic Missile Treaty on grounds that it curtailed the United States’ ability to protect itself from “terrorists” and “rogue states.”