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.