The American occupation of Montreal had been troubled from the start. After Richard Montgomery took the city without a fight in November 1775, relations with the local population deteriorated steadily. The Americans had interdicted the British fur trade on the upper St. Lawrence and the Great Lakes, severing an economic lifeline that affected supporters and opponents alike. By early 1776, reports reached General Schuyler of British and Indigenous military preparations to the west. When British Indian Department agent Claude-Nicolas-Guillaume de Lorimier slipped out of Montreal, the alarm deepened. Colonel Moses Hazen dispatched 400 troops under Colonel Timothy Bedel to fortify the Cedars. Lieutenant Isaac Butterfield led an advance party that arrived on April 26 and began building a wooden stockade, arming it with two four-pound cannons. Bedel and the rest of his regiment arrived on May 6. They were, in theory, ready.

Lorimier, the agent who had triggered the alarm by leaving Montreal, traveled to Oswegatchie where Captain George Forster of the 8th Regiment of Foot had occupied Fort de La Presentation. Lorimier proposed recruiting Iroquois warriors for a westward strike against Montreal. Forster agreed, and Lorimier went to Saint Regis, where he recruited 100 Mohawk warriors from Akwesasne. He arranged for a sympathetic priest near the Cedars to provision supplies and had soldiers of the 8th Foot hide shallow-draft bateaux at river crossing points. Forster departed Oswegatchie on May 12 with roughly 40 regulars, 10 Canadian militiamen, and 160 Iroquois warriors, picking up 44 more warriors at Saint Francis along the way. His total force numbered barely more than 250, yet it would prove enough. The Americans at the Cedars had the numbers, but their commanders lacked the nerve to use them.

Forster's force landed near the stockade on May 18 and demanded surrender. Butterfield asked to withdraw under arms; Forster refused. The two sides exchanged fire. The next day, Forster received reinforcements -- about 40 Canadian militiamen under Jean-Baptiste Testard de Montigny -- and learned that American reinforcements under Major Henry Sherburne had crossed the Ottawa River but retreated, believing the Cedars already fallen. When Sherburne reversed course and resumed his advance, Butterfield was unaware and surrendered the fort. The Haudenosaunee plundered the stores and claimed prisoners as war spoils, stripping them of valuables. Forster paid a ransom to prevent executions. The next day, Sherburne's 140 reinforcements were also captured after a brief skirmish. In two days, without a significant engagement, Forster had taken more than 500 American prisoners.

Arnold rushed back from Sorel and organized a relief force. He reached Fort Senneville on Montreal Island's southwest tip on May 26, burning it, just as Forster's men were landing on the far shore at Quinze-Chenes. Arnold wanted a surprise dawn attack, but Colonel Hazen, a veteran of the French and Indian War, argued fiercely against it. The war council voted Arnold down. Negotiations followed. On May 27, Forster -- whose force was shrinking as warriors drifted home -- agreed to a prisoner exchange through Sherburne and Butterfield. The American captives were returned at Fort Anne on May 30, delayed two days by high winds. But the Continental Congress repudiated the exchange agreement, accusing Forster of mistreating prisoners by placing them in Iroquois custody. No British prisoners were ever released in return.

Arnold blamed Bedel for the disaster. Both Bedel and Butterfield were removed from command and sent for court-martial, though the army's retreat from Quebec delayed the proceedings until August 1, 1776, at Fort Ticonderoga. Both were convicted and cashiered. Bedel continued volunteering, and after Burgoyne's surrender at Saratoga in October 1777, Congress granted him a new commission. Arnold's report of the affair included allegations -- never substantiated -- that two prisoners were killed by Forster's warriors. News of the engagement spread with wildly inflated casualty figures and graphic but false accounts of atrocities, shaping public perception far beyond what the actual skirmishing warranted. The site was designated a National Historic Site of Canada in 1928, marking the spot where a tiny force exploited strategic position, Indigenous alliances, and American indecision to humiliate the Continental Army's invasion of Quebec.