The Monroe Doctrine was signed on December 2, 1823 under President Monroe. It was written by the Secretary of State at the time, John Quincy Adams. It stated that any European involvement in North or South America would be seen as an act of aggression. It established both Americas as protectorates of the United States, and the Americas became part of the US' sphere of influence. This became the cornerstone of American foreign policy for a long time. It is similar to the defeat of China after the Opium war due to the idea of having a "sphere of influence" in a foreign country without actually conquering it.

http://attridgehistory.edublogs.org/files/2008/03/monroe-doctrine-1.jpg

The Roosevelt Corollary was added by President Theodore Roosevelt in 1904 as an addition to the Monroe Doctrine. It added onto the Monroe Doctrine, saying that the US had the right to force out Europeans that got involved in Latin American affairs. It was used as justification for the US' intervention in Cuba, Nicaragua, Haiti, and the Dominican Republic. I think it is somewhat similar to the Berlin conference that took place prior to the Imperialist conquering of Africa by many states, in the way that it dictates the terms of Imperialism.

http://en.wikipedia.org/wiki/File:Roosevelt_monroe_Doctrine_cartoon.jpg