The North Atlantic Treaty is the treaty that brought NATO into existence, signed in Washington, D.C on April 4, 1949.
The key section of the treaty was Article V which committed each member state 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 not invoked during the Cold War. Rather, it was invoked for the first time in 2001 in response to the September 11, 2001 Terrorist Attack against the World Trade Center and The Pentagon.
