In late June 1775, the Continental Congress, in hopes of adding a 14th colony and eliminating a British base for invasion, instructed Major General Philip Schuyler to take possession of Canada if "practicable" and "not disagreeable to the Canadians." Command of the main wing of the expedition, to march via Fort Ticonderoga to Montreal and down the St. Lawrence River, passed to Brigadier General Richard Montgomery when Schuyler became ill. A second wing, led by Colonel Benedict Arnold, was to move on Quebec through the wilds of Maine. It was an attempt by the Americans to take Montreal on September 25, 1775. From beginning to end, the defeat was a total fiasco. A small force of Canadian militia and Continental troops were organized and lead by a civilian adviser, future Vermont hero Ethan Allen. He had only been instructed to raise recruits for a Canadian militia and wait for an officer to take charge. Prior to that, Allen had been refused a commission in the American Army and voted out by his own militia, both for military incompetence. Lieutenant General George Washington wasted no time in exploiting this weakness and ordered Brigadier General Richard Montgomery to assemble a force to invade Canada. In August 1775, Montgomery did so with 2,000 New York and Connecticut militiamen, several hundred Canadian militia who wanted to make Canada a 14th rebellious colony, and Ethan Allen’s Green Mountain Boys.

Brigadier General Benedict Arnold, one of the victors at Ticonderoga, felt slighted that he wasn’t given command of the invasion so he left for Maine, then part of Massachusetts, to start his own invasion of Quebec. Ethan Allen and a small force of American and Quebec militia to capture Montreal from British forces on September 25, 1775, early in the American Revolutionary War. Allen, who had been instructed only to raise militia forces among the local inhabitants, had long had thoughts of taking the lightly defended city. When he reached the southern shore of the St. Lawrence River with about 110 men, he seized the opportunity to try. Major John Brown, whom Allen claimed was supposed to provide additional forces, did not appear as they had planned, isolating Allen and his men on the north side of the river. On September 24, during the night, Allen with 110 men crossed the St. Lawrence River, north of the town, but was left to fend for himself when Major Brown failed to meet him. Carleton, the British commander, sortied with a force of about 35 regulars, 200 volunteers, and a few Indians. Allen, unable to recross the river, took up a defensive position a few miles from the town. Most of the Canadian recruits fled when the first shots were fired, but Allen, constantly flanked by the Indians, led his ever-diminishing army on a fighting withdrawal for over a mile.

Finally reduced to 31 effectives and with a British officer "boldly pressing in the rear," Allen reluctantly surrendered. The abortive attack on Montreal led to the full mobilization of local militia in Montreal, raising nearly 1,000 men, but they soon began to drift away. Carleton refused to organize an expedition in relief of Fort Saint-Jean, and the militia members from rural parishes eventually disbanded to attend to their harvests and the defense of their own homes. This abortive attack on Montreal led to the full mobilization of local militia, which soon counted 2,000 men. But Carleton still refused to organize the relief of Fort St. Jean. Disgusted, the militias eventually disbanded to attend their harvests, and Carleton withdrew to Quebec. In November, Montgomery occupied Montreal without firing a shot. Thomas Walker, the merchant to whom Allen had applied for assistance, was arrested in early October 1775 when twenty regulars and a dozen militia came from Montreal to his house in L'Assomption. Walker's house was destroyed, and he was imprisoned with the intent of sending him to England for trial. Walker was eventually freed when the Americans captured Montreal and most of the British fleet trying to escape the city.