The north atlantic treaty organization (nato was formed in 1949 by the united states and representatives of western european states for the purpose of:

 NATO's original purpose was to serve as an alliance for defense against the Soviet-led communist block of countries.  

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After World War II, as the Cold War began, the the Soviet Union had shown that it wanted to expand its area of control in Eastern Europe.  In response, the United States, along with Canada, joined with ten European countries in signing the North Atlantic Treaty in 1949.  This created the North Atlantic Treaty Organization (NATO), which was a defensive military alliance of democratic states over against the expanding threat of communism felt in the Cold War environment.  The ten original Western European members of NATO were the United Kingdom, France, Belgium, the Netherlands, Denmark, Norway, Portugal, Italy, Iceland, and Luxembourg.

The formation of the Warsaw Pact was in direct response to the formation of the North Atlantic Treaty Organization (NATO). The Warsaw Pact was given that name because the agreement was signed in Warsaw, Poland.  Established in 1955, the Warsaw Pact included the Soviet Union, Albania, Bulgaria, Czechoslovakia, East Germany, Hungary, Poland, and Romania.  The nations signing the treaty called on each other to defend of any member of the Pact that was threatened by enemy forces.  

operation overlord was the codename for the battle of normandy, the allied operation that launched the successful invasion of german-occupied western europe during world war ii. the operation was launched on 6 june 1944 with the normandy landings