The Monroe Doctrine solidified the position of the United States as the dominant power in the Western Hemisphere. It became an integral component of American foreign policy.
The Monroe Doctrine was crafted by John Quincy Adams at a time when Spanish colonies in Latin America had begun to declare their independence. When President James Monroe declared the doctrine in his 1823 annual address to Congress, all foreign powers were given notice that any attempt to regain control of their former colonies would be viewed as a hostile act. The doctrine also had implications on North American soil. Russia was claiming vast swaths of land reaching as far south as modern-day Oregon. The candor of the doctrine made it clear that any foreign power would be prevented from expanding their presence in the area that would become the west coast of the United States. The doctrine can be considered successful in that no European powers intervened in South America, although it would be naive to suggest this was solely because of the Monroe Doctrine.
Decades later the Monroe Doctrine was a component of the manifest destiny policy that was used to justify western expansion. It was cited as a basis for interventionist policies of the United States well into the 20th century.