The Treaty of Brussels, signed on 17 March 1948 by Belgium, the Netherlands, Luxembourg, France and the United Kingdom is considered the precursor to the NATO agreement. The treaty and the Soviet Berlin Blockade led to the creation of the Western European Union's Defence Organization in September 1948. However, participation of the United States was thought necessary in order to counter the military power of the USSR, and therefore talks for a new military alliance began almost immediately.
These talks resulted in the North Atlantic Treaty, which was signed in Washington, D.C. on 4 April 1949. It included the five Treaty of Brussels states, as well as the United States, Canada, Portugal, Italy, Norway, Denmark and Iceland. Popular support for the Treaty was not unanimous; some Icelanders commenced a pro-neutrality, anti-membership riot in March 1949.
The Parties of NATO agreed that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all. Consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defense will assist the Party or Parties being attacked, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
Such action as it deems necessary, including the use of armed force does not necessarily mean that other member states will respond with military action against the aggressor(s). Rather they are obliged to respond, but maintain the freedom to choose how they will respond. This differs from Article IV of the Treaty of Brussels (which founded the Western European Union) which clearly states that the response however often assumed that NATO members will aid the attacked member militarily. Further, the article limits the organization's scope to Europe and North America, which explains why the Falklands War did not result in NATO involvement.
The creation of NATO brought about some standardization of allied military terminology, procedures, and technology, which in many cases meant European countries adopting U.S. practices. The roughly 1300 Standardization Agreements (STANAGs) codifies the standardization that NATO has achieved.
The first NATO Secretary General, Lord Ismay, famously stated the organization's goal was to keep the Russians out, the Americans in, and the Germans down.
The Warsaw Pact is the name commonly given to the treaty between Albania, Bulgaria, Czechoslovakia, East Germany, Hungary, Poland, Romania, and the Soviet Union, which was signed in Poland in 1955 and was officially called 'The Treaty of Friendship, Co-operation and Mutual Assistance'.
Nominally the Warsaw Pact was a response to a similar treaty made by the Western Allies in 1949 (the North Atlantic Treaty Organisation, or NATO) as well as the re-militarization of West Germany in 1954, both of which posed a potential threat to the Eastern countries. Although it was stressed by all that the Warsaw Treaty was based on total equality of each nation and mutual non-interference in one another's internal affairs, the Pact quickly became a powerful political tool for the Soviet Union to hold sway over its allies and harness the powers of their combined military. When Hungary tried to extricate themselves from the agreement in 1956, Soviet forces moved to crush the uprising; and, in 1968, Soviet troops invaded Czechoslovakia (with support from five other Pact members), after the Czech government began to exhibit 'Imperialistic' tendencies.
Following the diminishing power of the USSR in the 1980s and the eventual fall of Communism the treaty became redundant. The Warsaw Pact was officially dissolved in Prague in 1991, after successive governments withdrew their support of the treaty.