The Roosevelt Corollary was an idea by president Theodore Roosevelt that was based on the Monroe Doctrine. Basically, it established that the United States would be like a police force that would meddle in Latin American politics in order to prevent escalation or support a side in a war or something similar, instead of having Europe meddle in their affairs.
The Roosevelt Corollary was set forth by President Theodore Roosevelt in 1904 (in his State of the Union Address that year). It was a tweaking of the Monroe Doctrine. The Monroe Doctrine expressed US opposition to European interference in the Americas. Roosevelt's corollary to that doctrine was that the United States would intervene in conflicts between European countries and Latin American countries, rather than having the Europeans press their claims directly.
