The Cold War did not come to an end with a formal treaty. This is understandable because it was never a declared war. It was an ideological, economic, and political struggle between the United States and the Soviet Union. Although there were "hot" wars at times during the Cold War, Washington and Moscow never fought each other militarily. That is fortunate because both sides possessed large nuclear arsenals, and a conflict between the two would have been catastrophic—both for the two sides and the entire planet.
Historians disagree on the reasons why the Cold War ended. American conservatives give credit to President Ronald Reagan. They claim that Reagan's military buildup, which Moscow attempted to match, led to bankruptcy for the Soviet Union. Other historians argue that Mikhail Gorbachev, the last leader of the Soviet Union, was responsible for bringing it to a conclusion.
Indeed, Gorbachev's role was decisive. He implemented liberal reforms in the Soviet Union. Gorbachev withdrew Soviet troops from Afghanistan, and he signed arms control agreements with Reagan. As nations in Eastern Europe began to break away from Soviet control in the late 1980s, Gorbachev decided not to use force against them. Previously, Moscow had crushed revolts in Hungary (1956) and Czechoslovakia (1968), so his decision not to intervene was a clear departure from previous Soviet policy. Between 1989 and 1991, the Cold War came to an end with German unification and the disintegration of the Soviet Union.