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What was the main idea of the Monroe Doctrine?
It was a proclamation of war between the U.S. and Mexico.
It was a warning for European countries to stop colonization in the western hemisphere.
It was an agreement to give territories to the Native Americans.
It was a list of complaints from the U.S. to European countries.

Answer :

The main idea of the Monroe Doctrine was that "It was a warning for European countries to stop colonization in the western hemisphere," since the US saw such involvement as a threat to its national interests. 
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The main idea was "It was a warning for European countries to stop colonization in the western hemisphere.

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