The United States is a founding member of NATO , an alliance of 29 North American and European nations formed to defend Western Europe against the Soviet Union during the Cold War . Under the NATO charter , the United States is compelled to defend any NATO state that is attacked by a foreign power . The United States itself was the first country to invoke the mutual defense provisions of the alliance , in response to the September 11 attacks
September 11 attacks
September 11 attacks
Foreign policy of the United States