Welcome to Shattered Stars and Strips, a timeline where the United States doesn't stay united!
Divergence from OTL
In 1812, British General Isaac Brooks is not killed at the Battle of Queenston Heights, and is present to oversee the Battle of York. Under his leadership, the British repel the Americans at York and deny the US a major victory in the war. As a result of this stumble, the US also loses the Seige of Fort Meigs, and the Battle of Lake Erie. These losses just further degrade the creditably of the Federal Government. As a result, the ongoing Hartford Convention, led by Harrison Gray Otis, becomes increasingly radical, eventually becoming an advocates group for New England succession. Massachusetts Governor Caleb Strong,although not supportive of full succession, reaches out to the British to peruse a separate treaty, a slap in the face to the Federal Government. The attempt however, provides the more radical voices in the Hartford Convention the justification needed to draft a deceleration of independence. Strong, wanting only for the War to end and for Massachusetts to receive its lost land, backs the radicals and signs the deceleration, and in 1813 the Republic of New England declares itself independent from the United States. A subsequent invasion by the United States prolongs the war, and much of New England is occupied by the US. Receiving British backing, New England is able to liberate itself, and after a disastrous loss at the Battle of New Orleans, the US is forced to recognize the Republic of New England.
The Hartford Convention sets a precedent for state succession, and as a result the Unity of the United States is called into question, and with every crisis, the possibility for another succession arises.