Allen and his men crossed the St. Lawrence on the night of the 24th, landing at Longue-Pointe. The inhabitants he met there were friendly, but he posted guards on the road to Montreal to prevent news of their crossing from reaching the city. However, one man they detained managed to escape to the city, where Carleton was notified of Allen's presence on the island. Brown, for his part, did not cross the river, for reasons unknown. This left Allen's force alone and vulnerable, as it had taken three round trips with the available boats to ferry his men across the river.
Realizing he would not be able ferry everyone back across the river before troops arrived from the city, Allen chose a wooded area near the Ruisseau-des-Sœurs (labeled on the map above as Ruisseau de la Gde Prairie), between Longue-Pointe and Montreal, to make a stand. He also sent word to Thomas Walker, a British merchant and known Patriot sympathizer with a house in nearby L'Assomption, for assistance. Walker was able to muster some men, but Allen was captured before they could lend any assistance.
When General Carleton received word that the notorious Ethan Allen was at the gates of the city, he raised the alarm. As the news spread, large numbers of people turned out. Captain John Campbell eventually led a force of 34 regulars from the 26th Foot (the entire garrison in Montreal), 120 Canadians, 80 English, 20 Indian (native) agents, and a few natives, out to face Allen's force. As Campbell's troops approached, Allen instructed 10 Canadians to cover his left flank, while Duggan and another 50 Canadians were placed on the right flank. Both of these detachments fled, rather than holding their positions, leaving Allen with about 50 men. Over the course of about 90 minutes, fire was exchanged between the forces. Allen's remaining forces were eventually broken, and, after trying to outrun the enemy, he surrendered.
In late June 1775, the Continental Congress, in hopes of adding a 14th colony and eliminating a British base for invasion, instructed Maj. Gen. 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 Brig. Gen. Richard Montgomery when Schuyler became ill. A second wing, led by Col. Benedict Arnold, was to move on Quebec through the wilds of Maine.
Montgomery encountered strong resistance from the British at St. Johns, delaying his assault on Montreal. While undertaking a siege of St. Johns, Montgomery sent Col. Ethan Allen and Maj. John Brown ahead with separate detachments to recruit Canadian volunteers. They did manage to recruit some Canadians and, meeting together, conceived a risky plan for a converging attack on Montreal by 2 forces totaling about 300 men.
Allen's nominal objective was to secure the bank of the St. Lawrence River and to prevent British General Carleton from attempting a relief of Fort St. Jean, under siege by Montgomery. Acting on poor intelligence, the Americans decided to attack Montreal itself.
On September 24, during the night, Allen with 110 men crossed the St. Lawrence north of the town but was left to fend for himself when Brown failed to meet him. Gen. Sir Guy Carleton, the British commander, sortied with a force of about 35 regulars, 200 volunteers, and a few Indians, and 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.
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