The main principle behind the Monroe Doctrine, declared by President James Monroe in 1823, was to oppose European colonialism and interference in the affairs of the Americas. It asserted that the Western Hemisphere was no longer open to colonization and that any attempt by European powers to extend their influence or establish new colonies would be viewed as a threat to U.S. security. This doctrine established a framework for American foreign policy and demonstrated a commitment to protect newly independent Latin American nations, marking a significant shift in diplomatic relations.