Background
The conflict phase of the American Revolution had started with the Battle of Lexington and Concord in the spring of 1775.
After that the British Army was bottled up in the Siege of Boston, royal governors were being forced to leave the other colonies, and the American Congress had created the Continental Army.
Congress sought a way to seize the initiative.
Several times during the French and Indian Wars the colonies had either fought in Canada or been subjected to northern and western pressure from Indians supplied and provoked from there. The Congress authorized General Philip Schuyler, commander of the Northern Department to mount an invasion to drive British forces from Canada. He sent General Richard Montgomery north with an invasion force. General Washington also sent Benedict Arnold towards Quebec with a supporting force.
The Invasion
The Americans took the north end of Lake Champlain and Montreal.
By the end of 1775, they controlled the St. Lawrence River above Quebec, and had laid siege to the city and Canada's governor Guy Carleton.
But, this was the high water mark of the attack.
In a desperate, last effort to take Quebec, they launched the Battle of Quebec (1775) and were soundly defeated. Montgomery was killed, Arnold was injured, Daniel Morgan and Ethan Allen were captured. When General John Thomas arrived to take command and replace Montgomery, he found the army already decimated by the march north of the Arnold Expedition, smallpox, and the Canadian winter. He immediately started a withdrawal.
In 1776, The British in Canada were strengthened by troops under General John Burgoyne and Hessian mercenaries. The new American commander, General Thomas died of smallpox. With the additional troops, Carleton drove the Americans all the way back to Fort Ticonderoga in New York.