This is a policy adopted by the U.S. in 1823 whereby it decided to oppose colonial intervention into the Americas by European empires. It is named after American President James Monroe, who spelled out the doctrine in his State of the Union address to the U.S. Congress. The Monroe Doctrine was aimed at preserving the political independence that Latin American nations won from their European colonisers. It stated that any future European intervention in the Americas would be considered an act of aggression that would cause the U.S. to intervene in the matter.
