The Monroe Doctrine, declared by President James Monroe in 1823, stated that European countries could not colonize or interfere with independent nations in the Western Hemisphere. The doctrine aimed to prevent further colonization in North and South America by European powers like Britain and also keep the peace after costly wars like the Revolutionary War and War of 1812 drained American resources. The Monroe Doctrine helped maintain peace among existing nations in the Americas and protected American shipping and territory from European encroachment.