Answer: The Monroe Doctrine is the best known U.S. policy toward the Western Hemisphere. Buried in a routine annual message delivered to Congress by President James Monroe in December 1823, the doctrine warns European nations that the United States would not tolerate further colonization or puppet monarchs. The doctrine was conceived to meet major concerns of the moment, but it soon became a watchword of U.S. policy in the Western Hemisphere.
The Monroe Doctrine was invoked in 1865 when the U.S. government exerted diplomatic and military pressure in support of the Mexican President Benito Juárez. This support enabled Juárez to lead a successful revolt against the Emperor Maximilian, who had been placed on the throne by the French government.
Almost 40 years later, in 1904, European creditors of a number of Latin American countries threatened armed intervention to collect debts. President Theodore Roosevelt promptly proclaimed the right of the United States to exercise an "international police power" to curb such "chronic wrongdoing," in his so-called Roosevelt Corollary (or extension) to the Monroe Doctrine.
While the Monroe Doctrine’s message was designed to keep European powers out of the Western Hemisphere, Roosevelt would strengthen its meaning to justify sending the United States into other countries of the Western Hemisphere. As a result, U.S. Marines were sent into Santo Domingo in 1904, Nicaragua in 1911, and Haiti in 1915, ostensibly to keep the Europeans out. Other Latin American nations viewed these interventions with misgiving, and relations between the “great Colossus of the North” and its southern neighbors remained strained for many years.
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