The Battle of Alligator Bridge took place on June 30, 1778, and was the only major engagement in an unsuccessful campaign to conquer British East Florida during the American Revolutionary War. A detachment of Georgia militiamen under the command of General James Screven chased Thomas Brown's Loyalist company into a large position of British regulars established by British Major Mark Prevost and were turned back. General William Howe reluctantly agreed to support the third East Florida expedition, and in early April, Georgia's 400 Continental troops began to move south, occupying the site of Fort Howe on April 14. Over the next month, this force grew as Georgia militia and South Carolina Continental arrived, swelling the force to some 1,300 men by early May. On May 10, Howe arrived at Fort Howe and began organizing the march south. The conditions in the camp were unpleasant: the weather was hot, and there were frequent desertions. The expedition force finally began crossing the Altamaha on May 28, but moved very slowly, crossing the Satilla on June 21 and reaching the St. Marys River on June 26. Governor Tonyn and General Prevost were aware of the American progress. Brown and Indian forces continued to perform reconnaissance, occasionally skirmishing with the Americans and testing the security of their camps. Prevost moved some of his troops forward, placing most of them on the main route to St. Augustine. On June 30 Howe sent a force of 100 cavalry under James Screven south to locate Brown. Brown ordered a company of men to circle around behind them while the rest of his men hid along the road heading south from the fort.

The men Brown sent to flank the Continentals were betrayed by deserters and ambushed, with most of them captured or killed. Brown began moving down the road toward the Alligator Bridge, but was overtaken by Screven's company shortly before he got there. As a result, Brown's men were chased directly into the established British position at the bridge. There was some initial confusion, because neither Screven's nor Brown's forces had regular uniforms, so the British regulars thought all of those arriving were Brown's men. This changed quickly however, and a firefight broke out. Prevost's regulars quickly took up positions and began firing on Screven's men, while some of Brown's men went around to come at their flank. In pitched battle, men on both sides went down, Screven was wounded, and some of the Patriot militia narrowly escaped being trapped before Screven ordered the retreat. On July 1, Major Prevost moved out with his, Brown's and McGirth's men, and surprised a Patriot crew repairing a bridge. Rather than extending themselves, they then decided to retreat, felling trees across the road as they went. The divisions in the American camps, however, meant that there would be no further advance. The Continental forces were out of rice, and appealed to the Georgia militia for supplies. On July 6, the militia finally crossed the Saint Marys River, adding some strength to the Continental force, which had been reduced by disease and desertion to only 400 effective soldiers. The shortage of food and the ongoing command disagreements spelled the end of the expedition, which began its retreat on July 14. This effectively ended the idea in Georgia of gaining control of East Florida and left the area firmly under British control.