The most significant direct U.S. military intervention came in response to the Iraqi invasion of oil-rich Kuwait in August of 1990, which led to the Gulf War. Although the invasion didn't directly threaten American territory, a vital U.S. economic interest -- oil -- was at stake, along with principles of international law that protect the sovereignty and territorial integrity of nations.
The Korean War pitted the United States and the allies against the Soviet Union and its communist clients. The United States defended South Korea at the 38th Paralellel from the North Korean communist.
a policy set forth by U.S. President Harry S Truman on March 12, 1947 stating that the U.S. would support Greece and Turkey with economic and military aid to prevent their falling into the Soviet sphere.
It stated that further efforts by European countries to colonize land or interfere with states in the Americas would be viewed as acts of aggression requiring U.S. intervention. http://en.wikipedia.org/wiki/Monroe_Doctrine