1823 U.S. flag

In December 1823 President James Monroe delivered a message to Congress that would shape U.S. foreign policy for nearly two centuries. Known as the Monroe Doctrine, it asserted that the Western Hemisphere was no longer open to European colonization and warned that any attempt by European powers to extend their influence would be viewed as a threat to American security. At its core, it was less a detailed policy blueprint than a declaration of intent: the United States would guard its neighborhood.