We've all seen plenty of timelines featuring a United States that takes all of British North America, including Quebec, in either the Revolutionary War or another conflict with Britain soon theareafter, like the War of 1812, before conquering most or all of Mexico and then going on to annex much of the Caribbean, Central America and South America (not to mention other frequent targets of Ameriwank scenarios, like the Philippines and Liberia). But realistically, how much of the Western Hemisphere would the great powers of Europe allow the US to just have before seriously contemplating intervention?
It doesn't take a genius to look at a map and see that a country that controls both North and South America in its entirety has serious potential to become an empire that threatens the entire global balance of power. So where does Britain, France, etc draw the line? A complete conquest of Mexico? Some or all of Gran Colombia? Anything further south?