The Monroe Doctrine, on December 2, 1823, proclaimed the United States' opinion that European powers should no longer colonize the Americas or interfere with the affairs of sovereign nations located in America, such as the United States of America, Mexico, Latin America, and others. The United States planned to stay neutral in wars between European powers and in wars between a European power and its colonies. However, if these latter type of wars were to occur in the Americas, the United States would view such action as hostile toward itself.

The doctrine was issued by President James Monroe during his seventh annual State of the Union address to Congress. It was met first with enthusiasm. This was a defining moment in the foreign policy of the United States.

The doctrine was conceived by its authors, especially John Quincy Adams, as a proclamation by the United States of moral opposition to colonialism, but has subsequently been re-interpreted in a wide variety of ways, including by President Theodore Roosevelt as a license for the U.S. to practice its own form of colonialism (known as the Roosevelt Corollary to the Monroe Doctrine.)

The Monroe Doctrine states three major ideas, with one more added by President Theodore Roosevelt. First, it conveys that European countries cannot colonize in any of the Americas: North, Central, or South. Second, it enforces Washington's rule of foreign policy, in which the U.S. will only be involved in European affairs if America's rights are disturbed. Third, the U.S. will consider any attempt at colonization a threat to its national security. Roosevelt added to the doctrine, and summed up his additions with the statement, "Speak softly and carry a big stick."