North Atlantic Treaty

The North Atlantic Treaty is the treaty that founded NATO. It was signed in Washington, DC on April 4, 1949. 
Members.
The original twelve nations that signed it and thus became the founding members of NATO were the following:
Later the following nations joined:
When Germany was reunified in 1990, the country as a whole became a member of NATO.
Article V.
The key section of the treaty was Article V which says that each member state has to consider an armed attack against one state to be an armed attack against all states. The treaty was created with an armed attack by the Soviet Union against Western Europe in mind, but the mutual self-defense clause was never used during the Cold War. It was used for the first time in 2001 when the September 11, 2001 attacks against the World Trade Center and The Pentagon had taken place.
US-NATO relation.
In the United States, the treaty was approved by the Senate in a vote of 82 to 13 on July 21, 1949.