Fort Cumberland sat on the Isthmus of Chignecto, the narrow neck of land connecting Nova Scotia to the mainland. Originally built by the French as Fort Beausejour beginning in 1751, it had been captured by the British in 1755 and then essentially abandoned. By the time Colonel Joseph Goreham arrived with 200 Loyalist troops of the Royal Fencible American Regiment in the summer of 1776, the place was in deplorable condition. His men lacked food, uniforms, and basic supplies. The local population, many of them sympathetic to the Patriot cause, refused to help restore the fortifications and even tried to convince garrison members to defect. Goreham, a veteran of the French and Indian War, did what he could with what he had. He rebuilt palisades, mounted cannon, and waited for trouble he knew was coming.

Eddy's recruitment campaign was a study in persistent optimism against discouraging reality. He left Boston in September with a handful of volunteers, sailed to Machias on the Maine coast, and picked up about twenty recruits. At sea he encountered John Allan, his fellow Nova Scotian revolutionary, who tried to talk him out of the whole plan. The Mi'kmaq, the largest Indigenous nation in Nova Scotia, would not join, Allan warned. Eddy pressed on regardless, gathering nine Passamaquoddy warriors at Campobello Island and twenty-seven settlers at Maugerville on the Saint John River. At the Maliseet settlement of Aukpaque, near present-day Fredericton, the main chief Pierre Tomah declined to participate. A rival leader, Ambroise St. Aubin, agreed to bring fifteen warriors in exchange for the Maugerville community supporting their families. With about seventy-two men, Eddy headed for Fort Cumberland.

What happened next was audacious. On October 25, Eddy's men captured a British patrol at Shepody Outpost, killing one. More settlers joined at Sackville, swelling his force to around 180. Then came a stroke of luck: the supply sloop Polly arrived at the fort laden with winter provisions, and on November 6, thirty of Eddy's men surprised the sleepy guards aboard her. Thirteen prisoners were taken. The next morning, Goreham unknowingly sent another thirty men down to the dock, where they were captured one by one. With those sixty-plus prisoners went a quarter of the garrison and their critical supplies. By November 8, reinforcements from Cobequid and Pictou brought Eddy's force to nearly 400. He demanded Goreham's surrender. Goreham's reply was characteristically blunt: he suggested that Eddy surrender instead.

Without artillery, Eddy ordered a night assault on November 12. A feint was meant to draw defenders from the fort's weak points, but Goreham saw through it. One Maliseet warrior managed to sneak inside and nearly opened a gate before being stopped at the last moment. After the failed attack, Eddy lost control of his expedition as a council of leaders formed to overrule him. Night raids on November 22 and 23 succeeded in burning several buildings around the fort, but Goreham held his ground behind his patchwork palisade and six cannon. On November 27, HMS Vulture arrived with Royal Marines. Rather than retreat, the rebels dug in. At dawn on November 29, Major Thomas Batt led 150 men in a sortie that scattered Eddy's forces, killing and wounding several at a cost of two dead and three wounded.

The aftermath was swift and bitter. Homes and farms of rebel supporters were burned in reprisal, though British authorities took a surprisingly lenient approach toward captured rebels. Richard John Uniacke, one of the participants, went on to become Attorney General of Nova Scotia. Goreham offered pardons to anyone who surrendered their arms, and more than a hundred locals accepted. Eddy's scattered forces took weeks to reach safety; some Massachusetts men spent over two months struggling back to Machias through the winter wilderness. The battle's significance far exceeded its modest scale. By crushing the only serious military threat to Nova Scotia during the Revolution, the British ensured their Maritime colonies stayed loyal. The fort site is now a National Historic Site of Canada, administered by Parks Canada, where the earthworks still trace the outline of a place where the shape of a continent was decided by a few hundred men in a crumbling fort on a windswept isthmus.