This timeline is inspired by the great work of Thande, Drew, Glen, Eurofed, Wilcoxchar and a host of other contributors to these forums. I couldn't have started this without you.
That said, this is still a very rough draft and I want to get some feedback on it (especially from the aforementioned authors) so I can make it better and more plausible.
I'm going to post a brief history from the POD in 1752 until 1805, followed by maps of various dates in that range. I'll be the first to admit that I'm not the best mapmaker, and bear in mind that I haven't touched anything in South America or Europe since I've been solely focused on North America. I am interested in getting some help for how the POD and various butterflies affect these regions.
With that said, here we go.
Brief overview, part 1
(1755 through 1811)
In 1755, a man and a woman met by chance, and fell in love. Ordinarily this would not make a great difference in the grand scheme of things, but this romance was different for two reasons: he was William Shirley, the widowed governor of the Province of Massachusetts Bay; and she was Adèle Chartier, a young and very beautiful Acadian woman whose family had suffered during the siege of Louisbourg. Despite all odds, their romance blossomed and survived. When forced to choose between loyalty and love, the man chose love, and the consequences were disastrous, both for himself and his lover.
As a result, the war against New France took a very different course, one that turned more bloody as the Acadians were forewarned by the man and cleverly tried to elude capture, while the British general in charge of the war, Edward Braddock, grew ever more belligerent and vengeful. George Washington, a young lieutenant-commander from Virginia whose army was under the command of this general, learned much about the clever and agile tactics of the Acadians, before being dismissed from the war for objecting to the general's brutal tactics.
The general's dramatic death in 1759 also affected a young quartermaster-general in the British army named Guy Carleton, who eventually took up his cause; when he eventually became governor of Quebec in 1766, he took a harder line against the Quebecois than he otherwise would have taken, given his upbringing and experience prior to the war.
For the Acadians and their Quebecois compatriots, their ability to blend into the forests of New France (aided in no small part by their native allies) meant that most of their best fighters were spared from the Acadian Expulsion. They proved to be a clever and adaptable fighting force during the war, routinely sneaking past British lines to take control of cities which were formerly conquered by the British and left inadequately-guarded. Later, as the British assumed they knew what was going on in Quebec, these resistance fighters formed an effective "fifth column" against British rule and planned for a day when they could retake their lands.
As the new hard-line governor of Quebec, Carleton did nothing to allay the tensions simmering in the region once he realized the true scope of the resistance. Meanwhile, British political dissidents who had been "relocated" to a mild penal colony on the north shore of Lake Ontario began agitating for self-government. The Canada Act of 1774 (partially written by Carleton himself) upset just about everyone in North America except a few diehard loyalists. In 1775, the resistance in Quebec finally made their move and rebelled against British control.
Shortly thereafter, Patriot agitation reached a tipping point, and Loyalist forces retreated to "safe" forts while most of the American colonists rallied for independence. The war began in 1775, and the Declaration of Independence included signatories from Quebec and Nova Scotia (which was still nominally under Loyalist control) and found even more support due to British heavy-handed tactics. Loyalist support among the colonists quickly diminished as more commoners got enraged, and the few remaining Loyalists fled to the safe haven of Newfoundland.
The war proceeded badly for the British, who overestimated their support among the American colonists, and were still heavily reliant on the brutal tactics made popular under former general Braddock. The British failed to execute on several of their more clever advances, and were soundly repelled at most battlefields. Then the British shifted their efforts to the southern colonies, but their attempt to drive a wedge between north and south backfired severely and inadvertently paved the way for the gradual dissolution of the institution of slavery.
The final British assault in 1780 was an all-out campaign to destroy American cities on the Chesapeake Bay and the Potomac River. Last-minute support from France, Spain and the Dutch Republic helped to tip the scales in favor of the Patriots, and a huge battle in 1781 spelled the disgraceful end of the British attempts to retake the American colonies. The Treaty of Paris was signed in 1782 in which the Fifteen Colonies gained independence from Britain. However, Britain still had several forts and strongholds in North America, and would not give these up willingly.
After the war, the Congress of Confederation eventually reached an impasse and opened a new constitutional convention. However, negotiations broke down into a two-sided argument, with Quebec and its supporters against the rest of the former colonies. Differences proved to be insurmountable, and General George Washington ran afoul of both sides while trying to mediate the conflict; as a result, his reputation was tarnished and he quit politics in disgust.
Another moderate and intellectual delegate named Thomas Jefferson also got caught in the middle and quit for the same reason. Without these two moderates attempting to maintain order and find common ground, the resulting argument reached a fever pitch before Quebecois delegates walked out on the convention.
The remaining states managed to resolve their minor differences and pushed for a stronger federalist government than they normally would have taken, as a reaction against the more decentralized and state-focused approach sought by Quebec. The resulting Constitution included language which further increased federal authority and granted Congress and the Executive Branch the power to alter state borders, among other provisions. By sheer coincidence, the new Constitution was fully ratified on the same day that Quebec declared its autonomy from the United States.
When this news reached Philadelphia, the immediate reaction was angry shouts of betrayal, but the leader of the convention (and future first President) John Adams gained control of the chamber and threatened to execute anyone who would start another war on the continent. Meanwhile, Ambassador Benjamin Franklin was recalled from France and appointed as the first ambassador to newly independent Quebec, who expected an invasion force and were shocked to see a meek diplomat asking permission to cross their border and parlay.
Franklin eventually managed to repair relations between the two new nations; even though Quebec declined to rejoin the union, a treaty and several trade agreements were signed in short order, after careful negotiations. Both nations realized that they still lived under the threat of British invasion, so they vowed to support each other, which led to increased trade as well as some amount of migration in both directions across the new border.
Eventually Britain realized that its heavy-handed tactics had forged a bond between Quebec and the United States, and so wisely decided not to attempt an outright reinvasion. However, they did attempt to drive various wedges between the two nations and attempted to disrupt shipping with other European nations, at least until they ran into enough problems in their home island and later on within mainland Europe, including those caused by one Napoleon Bonaparte.
Distracted by looming wars on other fronts, Britain eventually agreed to sign a treaty in 1804 which gave both new nations ownership and control over most British possessions on the North American continent, but paved the way for further conflicts and wars in the coming decades. This treaty also put the United States on a direct collision course with the Empire of Spain, or to be more precise, its various successor states on the continent. It also dissuaded the United States from attempting to purchase the Louisiana region, and Spain was not really in a mood to sell it.
The resulting policy towards the creation of new States was affected by the perceived threat of attack by native tribes, and the more forceful but gradual resettlement of the same; the very real threat of marauding British forces stationed at forts which were supposed to have been surrendered long ago; and continued Spanish control over most of the nearby lands to the west of the Mississippi River. Increased federal power combined with these factors meant that many States were smaller and better-defended, with new states only being formed from territories when they were in a position to be defensible. The nation didn't want any new States to "bite off more than they could chew".
By 1805, the trends were clear. The United States was going to be comprised of appreciably more states, of average smaller size and somewhat less autonomy than IOTL. Settlement patterns and exploration to the west were likewise affected.