In November 1775, with Virginia tilting toward rebellion, Lord Dunmore issued a proclamation offering freedom to any enslaved person belonging to a patriot who would join the royal forces. The promise was strategic - it would weaken plantation owners and provide soldiers - but for the people who responded, it was something more. Hundreds slipped away from plantations and walked, ran, or sailed to Dunmore's ships. They were formed into the Ethiopian Regiment, given uniforms bearing the words 'Liberty to Slaves,' and trained for war. By December, Dunmore's forces had been defeated at the Battle of Great Bridge near Norfolk. The patriots took Norfolk; Dunmore burned what he could not hold on January 1, 1776, and withdrew aboard his fleet of nearly 100 vessels. With food blocked and disease spreading, he sailed thirty miles north and anchored at Gwynn's Island on May 26, hoping the place would let his exhausted force recover.

Dunmore inoculated his African-American troops against smallpox, and that part worked - smallpox itself did not destroy them. But a second illness, an unknown fever, swept through the camp. The water supply on the island proved inadequate for several hundred people. Captain Andrew Snape Hamond of HMS Roebuck, in command of the small naval squadron, reported that of the roughly 500 Black soldiers in the Ethiopian Regiment, only 150 survived. The Queen's Own Loyal Virginians were ill with smallpox; the British regulars of the 14th Foot were too weak to stand guard. Dunmore wrote to Lord George Germain in London that, except for the fever, he might have raised 2,000 Black recruits and ended the rebellion. He admitted that each of his ships was throwing one to three corpses overboard nightly. On the mainland opposite the island, the Continental soldiers watching from across the water saw bodies drifting in the bay. They took it as a good sign.

General Andrew Lewis arrived in the patriot camp on July 8, 1776, with a brigade of Virginia troops and the artillery that had been laboriously collected at Williamsburg. Two 18-pounder cannons were positioned opposite Dunmore's flagship, the Dunmore. Four 9-pounders covered the British camp and the small ships guarding Milford Haven. At 8 a.m. on July 9, the 18-pounders opened fire at a range of 500 yards. The first shot smashed through the Dunmore's stern and a splinter wounded the governor in the leg. The Dunmore's six-pounder guns could not reply effectively, and the demoralized crew flinched from the larger shot. With no wind, Dunmore ordered the anchor cable cut and the flagship towed out of range. The 9-pounders battered Fort Hamond and the British camp. After about two hours, the patriot artillery commander Captain Dohickey Arundel fired an experimental wooden mortar - which exploded on its first shot, killing him. He was the only patriot casualty of the entire engagement.

Dunmore and Hamond decided overnight that they could not hold Gwynn's Island. They loaded what they could - cannons, tents, supplies - and prepared to evacuate. On the morning of July 10, the patriot guns turned on the three vessels guarding Milford Haven, whose crews abandoned them. Lieutenant Colonel Alexander McClanahan led 200 men across to the island in canoes. The remaining loyalists fled to their ships and were gone by 1 p.m. What they left behind on the island was its own kind of horror: many sick and dying African-Americans, abandoned where they could not be moved. Captain Thomas Posey, one of the patriot officers who walked the island after the loyalists left, estimated that 400 to 500 Black people and 150 white loyalists had died during the six weeks of the encampment. The corpses of those whom Dunmore could not bury or burn at sea lay scattered across the sand.

Dunmore's surviving forces lingered in the Chesapeake into early August, raiding plantations along the Potomac. Patriots feared they would attempt to seize Martha Washington at Mount Vernon, but storms and local militia turned the raiders back. By early August, Dunmore's fleet rendezvoused at Lynnhaven Roads and dispersed. He sent ships to St. Augustine, Bermuda, and the Caribbean, loaded with roughly 1,000 enslaved people captured during the spring's raids. Dunmore himself sailed to New York, then to England, and eventually became governor of the Bahamas. The Virginia troops who had pried him off Gwynn's Island marched north to join George Washington's Continental Army. For the Black men and women who had joined Dunmore seeking freedom, the outcome was crueler. The lucky few who survived the fever and were aboard the departing ships scattered across the Atlantic - some ended up in Nova Scotia, some in the Caribbean, some still enslaved. The promise that had drawn them to Gwynn's Island was kept for almost none of them.