Monroe Doctrine

In 1823, the Monroe Doctrine pronounced that the United States would no longer interfere with existing European colonies in the New World and proclaimed that European powers were to leave American colonies alone. Then the U.S. took Florida from the Spanish and forged westward disputing with Great Britain over Oregon’s border, taking Texas in 1837, and New Mexico, Arizona, and California in 1848. When the Spanish-American war broke out in 1898, America leveraged Manifest Destiny to annex Guam, Puerto Rico, and the Philippines.

The Monroe Doctrine was an American policy formulated by President James Monroe in 1823 to limit European interference in the Western Hemisphere. The later addition of the Roosevelt Corollary basically put the Eastern Hemisphere on notice that the United States would do all the colonizing and exploiting in the Western Hemisphere from that moment on. The corollary was used to justify invasions of Haiti, Nicaragua and other countries, and to overthrow left-wing governments (even elected ones) with repressive, right-wing Juntas, and arguably spurred further anti-American sentiment. That the U.S. should act as the regional hegemon in the Western Hemisphere became so much a part of American political culture that events such as the overthrow of the governments of Arbenz in Guatemala and Salvador Allende in Chile were treated as unexceptional events by many American political elites.

The Monroe Doctrine is an expression of the longer-term geopolitical strategy of the U.S. often described as isolationism. The decision of the Truman administration to establish NATO and commit the U.S. to the defense of Western Europe, the decision of the Johnson administration to adopt Israel as a client state, and the decision of the Nixon administration to attempt to establish hegemony in the Persian Gulf each represent costly departures from the older geopolitical strategy of the U.S.

On the other hand
The early 1860s, a phase when the US was occupied with other matters, saw an unprecedented amount of war and European intervention in Latin America. Spain tried to re-colonise the Dominican Republic, France installed a Habsburg as Mexican emperor, and a great many other things that the US put an end to very soon after the civil war was over. So while the Monroe doctrine caused a lot of grief due to American intervention, no Monroe doctrine would have meant more European intervention.