The Monroe Doctrine is nominally an American guarantee of sovereignty to the independent nations of the Americas, but really from the get go it was a claim to hemispheric dominion (all “guarantees of sovereignty” basically are, true sovereignty guarantees itself) as the successor to Spain.

It was originally proclaimed in the context of the ultimately successful independence movements of Spain’s various continental possessions, as local elites took advantage of Spain’s Napoleonic-era vulnerability to carve out their own feifdoms.

This doctrine effectively claimed the Americas as part of the US economy and sphere of influence without the necessity of formal annexation (as direct integration of the Catholic, Hispanophone, feudal, indigenous and hybridized, cultures of South America into America was too big an ask for a United States that was still struggling to reconcile its own northern mercantile republicanism and southern extractive feudalism.)

The young empire of America got away with it mostly because it wasn’t worth the bother for anyone else to contest the claim - inland South and Central America were largely too malarial or, on the Pacific side, mountainous for intensive development; extractable or already extracted precious metals had largely been exported to Europe already (prompting inflation which reduced the value of further extraction); the railroad, steamship, and refrigeration infrastructure that would enable the later perishable fruit trade was yet to be developed.