The Monroe Doctrine was the declaration by President James Monroe, in December 1823, that the United States would not tolerate a European nation colonizing an independent nation in North or South America. Any such intervention in the western hemisphere would be considered a hostile act by the United States, though the United States would respect existing European colonies.
What prompted Monroe’s statement, which was expressed in his annual address to Congress (what today would be considered the State of the Union Address) was a fear that Spain would try to take over its former colonies in South America, which had declared their independence.
It was believed that France, which had invaded Spain and restored its former king to the throne, was behind Spanish intentions to become involved again in South America.
The European powers took note of Monroe’s declaration, but what kept the Spanish (and presumably the French) from meddling in the western hemisphere was not so much Monroe’s statements as very real threats from the British. It seemed apparent that the Royal Navy would stop the Spanish involvement, as the British wanted to protect their interests in the Caribbean.
The Monroe Doctrine, although named for President James Monroe, was really the idea of John Quincy Adams, the future president who was serving as Monroe’s Secretary of State.
And while it wasn’t thought to be terribly important at the time, it was later invoked by other presidents. And the idea that European powers should not interfere in the western hemisphere became an important part of American foreign policy.