A brief examination of the history of European colonization of the Americas is necessary to understand President James Monroe’s motivation for issuing the foreign-policy statements that came to be known as the Monroe Doctrine. In 1823, the United States had been an independent nation for only about half a century. Mexico had won its independence from Spain only two years before, in 1821, and a wave of revolutions in Central and South America had just begun to overthrow European colonialism. In this context, the United States—itself still a young and vulnerable republic—had reason to fear that any further European colonization or re-colonization of territories in the Western Hemisphere represented a threat to its own independence. In declaring the Western Hemisphere an American sphere of influence, Monroe hoped to preempt European ambitions in the region. 

The Age of Discovery and Conquest began in the late 15th century when Portugal and Spain effectively split the world between them for exploration and colonization through the Treaty of Tordesillas (1494). Britain, France, and the Netherlands followed soon after. In the 17th century, Britain and France controlled most of North America, with Spain dominating in Central and South America and the Caribbean.