In December of 1775 American forces under Montgomery and Arnold attacked the city of Quebec. The attack itself ended up failing largely because of 1). Deserters informed the defenders in the city of the coming attack, and 2). Montgomery's death at the beggining of the battle.
So let's say that those two points don't come to fruition. So Montgomery lives, and let's say that either no one deserts, or the deserters don't reach Quebec City for whatever reason. What happens next? Do the Quebecois support the Americans? How does Britain respond? How does France respond? Does this gaurentee an American victory, does it motivate the British to raise the American colonies to the ground, or do the British go to the negotiating table?