Allen had been sent south of Montreal by General Richard Montgomery with modest orders: join James Livingston's Canadian militia and secure the south bank of the St. Lawrence to prevent British General Guy Carleton from relieving the besieged Fort Saint-Jean on the Richelieu River. Allen was not supposed to attack Montreal. But Allen had harbored the goal of taking the city since his triumph at Ticonderoga, and when he met Major John Brown at Longueuil, the two hatched a scheme. Brown would cross the river with 200 men at La Prairie, upstream from Montreal. Allen would cross at Longueuil with his Americans and 80 Canadians under captains Loiseau and Duggan. After a prearranged signal, both forces would converge on the city. It took three round trips in the available boats to ferry Allen's men across the river. Brown and his 200 men never crossed at all.

Allen's force landed at Longue-Pointe on the eastern end of the island of Montreal -- then a rural area, today the Mercier-Est neighborhood of Mercier-Hochelaga-Maisonneuve. The local inhabitants were friendly, and Allen posted guards on the road to Montreal to prevent word of his crossing from reaching General Carleton. But one of the men they detained managed to escape and made it to the city, where he told Carleton that the notorious Ethan Allen was at the gates. Allen quickly realized the problem. He could not ferry his men back across the river before British troops arrived, and Brown's promised force was nowhere in sight. He chose a wooded position near the Ruisseau-des-Soeurs, a creek between Longue-Pointe and Montreal, and sent word to Thomas Walker, a British merchant and known Patriot sympathizer in nearby L'Assomption, begging for reinforcements. Walker gathered men, but it was already too late.

Captain John Campbell marched out of Montreal with the entire British garrison -- 34 regulars from the 26th Foot -- supplemented by 120 Canadien militia, 80 English militia, 20 British Indian agents, and a handful of Indigenous allies. It was a force of roughly 250 against Allen's 110. Allen positioned his men for a stand: 10 Canadians on his left flank, Duggan with 50 Canadians on his right. As Campbell's force approached, both flanking detachments broke and fled, leaving Allen with approximately 50 men. The skirmish was brief and decisive. Allen and a number of his men were captured. The folk hero of Ticonderoga was now a prisoner, standing before Colonel Richard Prescott in Montreal.

Allen was imprisoned in a ship's hold and sent to England. He spent about a year on prison ships before being paroled in British-occupied New York City in October 1776 -- the British authorities feared that hanging him would create a martyr. He was finally exchanged in May 1778 for British Colonel Archibald Campbell, and returned to military and political service in the nascent Republic of Vermont. His fellow captives fared differently. When Prescott threatened to execute the captured Canadiens who had fought alongside him, Allen interceded: "I am the sole cause of their taking up arms." Thomas Walker, the merchant who had tried to send help, was arrested in early October 1775 when soldiers came to his house in L'Assomption. His home was destroyed, and he was imprisoned with the intent of shipping him to England for trial.

Allen's failed attack produced an ironic result: it helped the Americans take Montreal. Carleton's response to the raid -- mobilizing nearly 1,000 local militia -- looked decisive but proved unsustainable. The militia members from rural parishes drifted away within weeks, returning to their harvests and the defense of their own homes. Carleton refused to organize a relief expedition to Fort Saint-Jean, and when the fort's commander capitulated in November, the road to Montreal lay open. General Montgomery occupied the city without firing a shot on November 13, 1775 -- less than seven weeks after Allen's debacle. Today, a city park in the Mercier-Hochelaga-Maisonneuve borough marks the spot where Allen made his stand. It is called Parc de la Capture-d'Ethan-Allen -- a name that memorializes not a victory but a capture, which is exactly the kind of honest commemoration that Allen, who wrote his own colorful memoir of the affair, might have grudgingly respected.