As articulated in 1823, the Monroe Doctrine laid out four basic tenets that would define U.S. foreign policy for decades. The first two promised that the U.S. would not interfere in the affairs of European states, be they wars or internal politics, and that the U.S. would not interfere with European states’ extant colonial enterprises. In exchange, it stipulated that the Western Hemisphere was no longer open to further colonization and that any attempt on the part of a European power to colonize territory in the Western Hemisphere would be understood by the U.S. as an act of aggression.