i know it's a long shot, but what if Britain never joined the anti-napoleon coalition and a stronger napoleon tried to expand french Louisiana westward threatening the US instead of invading Egypt. a united states already weakened by war of 1812 tries to launch a preemptive attack against napoleon, but it is badly organised and fails disastrously(unrealistic i know). this defeat teamed with weakened economy as a result of french blockade leaves to states becoming less confident in the union. the British government (who would be looking fr an excuse to weaken napoleon so as to maintain the European balance of power) promises some states to send navy to brake blockade if states agree to rejoin empire while still retaining low taxes and a degree of autonomy. some states agree, (most likely from the industrialized north as the British empire was very anti-slavery at the time) and the British destroy napoleons fleet in an alternate american battle of the Nile while Canada along with Iroquois allies who have lost land to napoleon help pro British states. and with a heavy British military presence in the area as well as good public opinion of Britain due to the war, the american government have to let these states leave. over the next decade they are easily assimilated into Canada. for the sake of this timeline we will assume that everything north of new York goes Canadian.
once Napoleon was defeated at Waterloo the Americans are no longer able to play off the British and Americans against each other and a stronger Canada wouldn't allow the Americans to expand west into Iroquois lands or french Louisiana which they would have officially claimed after the Napoleonic wars, although it will still be primarily be occupied by native Americans.
Over time the British will refuse to trade with the Americans and British navy will prevent the US from trading elsewhere and the Americans (whose income comes almost entirely from selling cash crops to Europe). additionally the British may try to support slave revolts or help free slaves in america similarly to the union during the civil war in our timeline. once america is on the verge of collapse the British will offer the Americans an offer they can't refuse, if they agree to accept the British monarch as head of state as well as respecting everything west of the Appalachian mountains as crown land and agree to abolish slavery,in return they will not be taxed by Britain and will be self governing on civil affairs, similar to Australia or Canada today, and again more states will join the empire. while some of the southern states may still refuse, by this point the Canadians will be militarily vastly superior to the US an d this timeline could go one of two ways
1. Britain declarers war on the US, this will go similarly to the US civil war except that with the help of the invincible British navy the union will have even more of an advantage, however the south cold resort to guerrilla warfare making this conflict long and bloody and it will probably still be a sore subject to this day with the south considering the north traitors and the north considering the south violent racists, kind of like civil war resentment now, but worse.
2. Britain continues to put economic sanctions on the south unless they abolish slavery, eventually slavery will not be worth it and the south will probably abolish it sometime in the 1850s. by this point the us and Canada have had 70 years to grow apart culturally and it is unlikely that they will reunite without a good reason. instead the south will become a sort of small Canadian puppet state similar to what Canada is to the US today. it is possible that this southern state could make an alliance with mexico and side with the central powers in world war 1 to reclaim their lost lands, but this is unlikely and if it did happen, the north would probably win and may annex the south.
In the long term this reconquista will probably mean that
a. the British government will know that a larger US will be more likely to rebel again, so they will probably make sure that Canada/America never expands as far west and many native american nations survive to this day, while heavily under Canada's sphere of influence
b. as the south had to give up slavery earlier, it will probably be more industrialized.
c. as the colonies of Russian Alaska, and Spanish California were both very weak, it is possible that japan may try to colonise the west coast, alternatively, after Italy and Germany unify in the 1860s and start looking for colonies, they may try to found their own colonies in america bringing them into direct conflict with Canadian allied native Americans and speeding up the arrival of world war two.
d. as the us/Canada would not have a large pacific presence, pearl harbor would never happen and instead all american forces would be dedicated to fighting Hitler from the start of world war two. while Britain may try to reclaim their Asian colonies after world war two, more likely it will begin a sort of cold war with the Japanese, funding local resistance movements, and eventually the Japanese empire would collapse as a result of ethnic tensions in the area.
e. american army being on western front from start of world wars one and two will mean that Germany is defeated quicker, and in the case of ww2, the western powers would liberate most of eastern Europe, so the eastern Bloc would never form, and the cold war would be much smaller
f. as Britain would be much stronger during the late 1800s, it is possible there will be a balance of power war against Britain, similar to what happened to France in the Napoleonic wars, to Germany in ww1,or to the Hapsburgs in the war of Austrian succession and the thirty years war
