The Monroe Doctrine is a United States policy first announced in 1823 that asserts that the Western Hemisphere is off-limits to intervention from external powers. The Doctrine also states that the U.S. would regard any further European colonization or control of the Americas as an act of aggression requiring U.S. intervention. The Monroe Doctrine was issued at a time when the U.S. was concerned about the expansion of European powers in the Americas and wanted to ensure its own security and influence in the region.