Original: The Monroe Doctrine , expressed in 1823 , proclaimed the United States ' opinion that European powers should no longer colonize the Americas or interfere with the affairs of sovereign nations located in the Americas , such as the United States , Mexico , Gran Colombia and others .
Simplified: The Truman Doctrine by President Harry Truman during the Cold War stated that the United States will give aid to Turkey and Greece to help them resist the spread of communism .