North Atlantic Treaty Organization (NATO)
a military alliance among twenty-eight countries on both sides of the Atlantic, from the United States and Canada to Turkey. Established in 1949 by the United States, Canada, and ten western European countries, NATO—sometimes known as the Atlantic Alliance—has grown over the years to include many former states of the Soviet Union. Article 5 of the treaty that created NATO establishes its core principle of “collective defense,” which commits member countries to defend each other if attacked.