Washington is killed in a battle with Shays rebellion. It spreads and overthrows the state of Massachusetts.
The USA descends into anarchy with multiple states and factions vying for power.
The UK intervenes but fails to reconquer the USA.
The 13 colonies eventually settle into their borders as independent nations, all of them expand westwards somewhat and fight wars with natives.
Mexico rebels from Spain around the OTL time but becomes an Empire rather than a republic. They choose a Habsburg monarch.
With no powerful rivals on the continent, Mexico becomes the primier power in North America, siding with Britain during the Napoleonic Wars and invading Louisiana.
They discover the California gold and grow rich off of it. By the middle of the century California is colonized with Mexicans and the country has a transcontinental railroad.
A war with the UK over a Mexican siezure of Panama in the late ninteenth century results in a Mexican Victory, a Panama Canal by 1900, and enmity with Britain.
Mexico joins the first world war on the side of the central powers and invades Belize and Canada.
The British are sufficiently distracted that not enough aid comes to France, and the battle of Ypres is won by Germany. France collapses and Russia falls to revolution after Germany and AH destroy its armies.
Mexico gets British Columbia, Belize, and Russian Alaska. A world war two between the CP (plus some US states) and a revived entente (revanchist France, the RSFSR, the UK, Japan, and some of the USA factions) results in Germany nuking London, and the British Empire collapsing. All of North America is conquered by the Empire of Mexico.
And everybody liv4ed happily ever after.
lol just kidding, this was a joke.