I think you just didn't look in the right place for the answer. Here's what Wikipedia has to say (I found this in 5 seconds):
The Monroe Doctrine was a United States policy that was introduced on December 2, 1823, which said that further efforts by European governments to colonize land or interfere with states in the Americas would be viewed by the United States of America as acts of aggression requiring US intervention.[1] The Monroe Doctrine asserted that the Western Hemisphere was not to be further colonized by European countries, and that the United States would not interfere with existing European colonies nor in the internal concerns of European countries.