The Monroe Doctrine, promulgated Dec. 2, 1823, stated that intervention by European monarchies in the democracies of the New World would be judged as "the manifestation of an unfriendly disposition toward the United States." It also pledged that the US would not interfere in the affairs of European colonies in the New World.
The Doctrine had little immediate effect, but had dramatic impact later on. Initially, the Doctrine did seem to deter European efforts to stop independence movements in the Americas, leading to a profusion of newly independent states. In 1898, uprisings in Cuba, Puerto Rico and the Philippines against Spanish rule, immediately brought the Monroe Doctrine into play. Shortly after the destruction of the USS Maine, the US declared that any further Spanish action against the uprisings would be considered to be direct violation of the Monroe Doctrine, and thus began the Spanish-American War.
Theodore Roosevelt greatly expanded the purview of the Doctrine with his corollary, "Talk quietly and carry a big stick." This model of foreign policy, essentially made the US a sort of "Policeman" to the Americas, and led to multiple US involvements in Latin American affairs; most notably Panama's independence from Colombia, and its influence on the building of the Panama Canal.
Even today the effects of repeated interventions in Latin American affairs can still be felt. The late Hugo Chavez put Venezuela on a path of hostility towards the US. Raul Castro maintains hostility to the US, similar to his brother Fidel. US-Nigaraguan relations tread on egg shells. Even countries that have relatively friendly relations with the US e.g. Costa Rica and Chile, do have pockets of anti-American sentiment.