The principle, first stated by President James Monroe in an 1823 message to Congress, that the Americas were closed to further European colonization and that Europe must not extend its political system into the hemisphere. Militarily a bluff when issued — enforced in practice by British sea power — it hardened as the United States grew strong into a claim of hemispheric dominance, extended by the 1904 Roosevelt Corollary and read across Latin America less as a shield than as the charter of a new master.
In December 1823, a president buried a few sentences in a long message to Congress. The Americas, James Monroe wrote, were no longer open to European colonization, and any attempt to extend Europe's system here would be seen as a danger to the United States. At the time it was close to a bluff. The young republic had little power to stop a determined European expedition, and British sea power, guarding its own trade, did much of the actual deterring. But the words outlived their weakness. As the United States grew into a great power, the warning meant to shield new neighbors became the charter by which Washington claimed the whole hemisphere as its own backyard. The same passage, read in 1823 as a promise of protection, was read a century later, across Latin America, as the announcement of a master. Almost nothing in it named the United States as ruler, and yet that is what it became.