What is the Monroe Doctrine and its significance?

The Monroe Doctrine was used as a way of allowing the
United States to be aggressive in terms of colonialization or the manipulation of
American countries while suggesting that European nations ought to really butt out.  In
fact, if they didn't butt out, their actions would be seen as acts of aggression towards
the United States to be answered by whatever means the U.S. felt
appropriate.

In practice, the British were actually
somewhat in favor of it as it really went along with their economic interests in the
region so their very powerful navy helped to enforce the doctrine by containing the
Spanish and some other European efforts in the area.  But the doctrine really became an
excuse for the U.S. to do what it wanted rather than a document helping to outline and
encourage action by the Americas as a whole.