Your challenge is to have a complicated Canadian civil war, the PoD can be anywhere in the 20th century. One possible kickoff point would be if the Quebecois chose to have a war of independence.
I think this scenario is unlikely, but...

"Oui" wins in '95, negotiations break down, Parizeau declares unilaterally, Canada announces they will give immediate recogniton to any and all First Nations making similar declarations against Quebec("Big shout out to all our Mohawk friends!"), and the good times roll.

In any case, it would probably have to be something like that, with that general cast of characters. There aren't many Canadians who feel strongly enough any given issue to take up arms for an extended period.
An American civil war is almost certainly going to spill over into Canada. With Canda depending so much on US trade chaos in the states will cause chaos in Canada as well with mass unemployment and lots of very angry people, there's going to be millions of well armed refugees on top of that with all the unpleasantness of trying to distribute dwindling resources and food, and i wouldn't bet on US combatants respecting the border and not recruiting Canadians, voluntaryily or otherwise.