The Monroe Doctrine was important because it stated that the newly independent United States would not tolerate European powers interfering with the nations in the Western Hemisphere, and if the European powers did interfere, then the United States would retaliate with war. This strongly worded document was presented before Congress on December 2, 1823 by President James Monroe.
The president's secretary of state, John Quincy Adams, was also responsible for helping to shape the message of the statement. It was influential because it addressed both the Russian czar's proclamations, and the revolutions in Central and South America. Monroe and Adams refused to accept the Russian czar's claims to land on the American continent. They also wished to support the revolutionaries in Central and South America, since they worried that if France and Spain tried to reassert their rule, then they might attempt to take back the Americas as well.
Although the European powers did make military interventions in Latin America from time to time after the Monroe Doctrine was announced, the Americans did not look for war. They did, however, use the doctrine as justification for taking Texas in 1842 under President John Tyler. They also would use the doctrine again in 1890 to help decide how to settle a dispute between Great Britain and Venezuela over the British Guiana boundaries.