I've heard the phrase 'Monroe Doctrine' many times before reading about James Monroe, but never had a full understanding of what it meant.
In the political climate of the Cold War in the twentieth century or the current War on Terror, it was usually thrown out there to indicate the the US has a precedent for proactively attacking forces hostile to it to defend its interests.
In the Cold War, it was often used to justify invading countries that could be overthrown by our Soviet Rivals. In the War on Terror, it's been used by the Bush administration to justify overthrowing the Iraqi government to prevent that country from potentially harboring terrorist elements.
This isn't a value statement on the Cold War or the War on Terror, that's for another post, but it should be noted, that both these representations of the Monroe Doctrine are completely misguided.
The Monroe Doctrine was essentially a belief that Old World powers were no longer to be accepted in North America with the exception of British Canada. At the same time, it was a belief that America should not meddle in European affairs and wars of intrigue.
This policy of self defense at home and neutrality in abroad is the opposite from proactively attacking those hostile to us abroad that characterized the Cold War and the current War on Terror.
The Monroe Doctrine fit perfectly with protecting a still ascendant but not yet world power of America from the disastrous European wars that were tearing apart the continent and threatened to undermine our sovereignty. It was not a policy of American imposing its will on the world to protect its interests and was implemented at a time when very recently our Capital had been in flames.