The main purpose of the formation of the North Atlantic Treaty Organization (NATO) after World War II was:
1. **To set up an alliance of nations willing to defend each other should it be necessary:**
- NATO was created in 1949 with the primary goal of providing collective security against the perceived threat of the Soviet Union and its expansionist policies during the Cold War.
- Member countries of NATO agreed that an attack against one member would be considered an attack against all, thereby establishing a system of mutual defense.
This collective defense principle was a key aspect of NATO's formation and remains a central tenet of the organization.