The Monroe Doctrine was a cornerstone of the United States foreign policy, stated in 1823 by President James Monroe. It viewed as a hostile act future increased colonialism and control by European power in the western hemisphere, the United States in return not interfering with existing European colonies, internal affairs, and wars.
It may have reduced European imperalist interventions in the western hemisphere during the "Age of Imperialism", possibly also contributing to American non-interventionism, although not preventing American intervention in Europe in WWI.
There are various criticisms, such as in effect being a declaration that the western sphere was part of a US imperial sphere of interest, where the United States itself was not excluded from making interventions.
One example of invoking it was during the Spanish–American War, although one effect was Spanish colonialism being replaced with American colonialism.
The Monroe Doctrine is sometimes still invoked, ignoring the part of it stating that the US should not be involved in Europe.