OK, I've charted out (speculatively) the course of the war, through a series of maps of incalculable awfulness, written up on Paint. Just use them as rough guides, and avert your eyes if possible. Some of what I've said contradicts what Landshark has said, but it ends in reasonably the same position.
OK, here we go:
1997
New England and Quebec invade New York; New England after a border incident and because of historic rivalries, Quebec to stop New England from getting there first.
1998
Vermont Revolt begins, with partisan fighting from the so-called VC, Vermont Comms, disrupting New England war efforts. Quebec and New England are still allied, but this is becoming increasingly fragile.
1999
Quebec declare war on New England over division of New York's territory. VC rebels seize Montpelier and much of southern Vermont, and declare the Second Vermont Republic. Elections are held, with the Vermont Socialist Party (the political wing of the secessionists) beating the Liberals (former New England Whigs, who dominated New England politics; the Whig Party in Vermont split, with the main body boycotting the elections), against allegations of vote-rigging. See map.
2000
The fighting in Maine forces both Quebec and New England to scale down their commitments in New York. New York launches a counter-offensive into Connecticut, due to the drain on New England resources by the continuing wars against Vermont and Quebec. See map.
2001
The Vermont Republic is defeated, with Governor Dean, leader of the Socialist Party, killed in the Battle of Montpelier. The casualties incurred, however, prove disastrous, as Quebecois and New Yorker forces advance deeper into New England. The winter, however, proves decisive, as the badly-equipped New Yorker and Quebecois forces cannot keep up their logistics lines. In the Battle of Boston, both armies are defeated by the combined German-British forces, and forced to retreat. To make matters worse, the Canadian forces, previously deployed in Pacifica, launch a massive invasion of Quebec at the Battle of Montreal. See map.
2002
Quebec is rapidly forced out of New England. New Yorker forces are forced into a long retreat by the regrouping New Englanders. See map.
2003
Quebec falls to Canada. New York is finally expelled from New England. The strain of the war, coupled with its failure to retake New York City, means its economy is on the point of collapse. See map.
2004
New England launches a massive invasion of New York, and is only stopped once the angry British Commonwealth, now free from its Quebec commitments, forces them to halt their advance through force. However, to prevent a war with German forces (who are aiding New England), the British do not press their advantage further. See map.
2005
The Treaty of San Francisco is signed. Since New England are the victorious power, New York admits its war guilt, even though this is disputed. New England is forced to pay reparations to Britain, but keeps its captured territory to avoid a British-German war.
Yes, I know much of what I've said doesn't stack up with what's been discussed, but I think it's reasonably in line with the thread.
Now, for the pictures. The horrible, horrible pictures. Which I'll space out over several responses.