Canada goes with the rest of the colonies in the revolution. Later the US buys Louisiana like OTL, colonizes the Oregon Territory, Buys Alaska, and takes Florida, Cuba, Jamaica, Haiti, and Puerto Rico. Meanwhile Mexico revolts, manages to be even more successful and takes everything down to Panama, and later annexes Hawaii, and takes Spain's colonies in the Pacific.
While this has been going on both nations have had an uneasy peace with disagreements over where the border is resulting in some skirmishes that leads to a couple of wars that end inconclusively due to their relatively equal strength. This results in both nations reaching out to Europe for Allies in the late 1800s. Mexico befriends France and Russia which leads the US to ally with recently formed Germany. When the alliance drags everyone into a great war, that started over Russia invading Sweden due to it supporting Finnish separatists and Germany backing Sweden. The sides are the Entente (France, Russia, Britain, Denmark, Spain, Portugal, Brazil, and Siam) and the Allies( Germany, the United States, the Netherlands with Belgium, Austria, Sweden, the Ottomans, Argentina-Chile, and Japan). The largest front is the American front starting from the Texas-Louisiana border to the California-Oregon border resulting in thousands of miles of trenches that split the continent in two.
If anyone knows how this would go please run with it.