Following World War II, ideological, economic, and political differences between the United States and the Union of Soviet Socialist Republics led to mounting tension and hostility, and advances in nuclear technology deepened the strain as both nations raced to expand their nuclear capabilities. The theory of deterrence emerged, holding that possessing nuclear weapons was necessary to prevent either side from using them, and the Minuteman missile system became part of the United States' deterrent force. Construction of the first of six Minuteman Missile Wings began in 1961, and by 1967 construction was complete, with 1,000 Minuteman missiles on alert in underground silos across the Great Plains; the November-33 Launch Facility and Oscar-Zero Missile Alert Facility were part of the 321st Strategic Missile Wing at Grand Forks Air Force Base, the final wing constructed. Earlier arms treaties and agreements did little to halt the growth of nuclear stockpiles, but on July 31, 1991, U.S. President George H.W. Bush and Soviet President Mikhail Gorbachev signed the Strategic Arms Reduction Treaty, which drastically limited strategic offensive arms on both sides and became the most comprehensive arms treaty to that date. Its requirements made reductions in the Minuteman force imminent, and the Base Realignment and Closure Commission recommended deactivation of three Minuteman missile wings, including the 321st; by June 1998, the final missile in the 321st had been pulled from its silo. START also established extensive inspection and verification measures to ensure compliance, allowing treaty inspection teams at inspectable sites in the former Soviet Union and the United States to make close observations, take measurements, and take photographs when questions arose; as of August 15, 2009, the United States had conducted 647 inspections and Russia had conducted 470.