The siege of Fort Stanwix (also known at the time as Fort Schuyler) in 1777 began on August 2 and ended August 22. Fort Stanwix, in the western part of the Mohawk River Valley, was then the primary defense point for the Continental Army against British and Indian forces aligned against them in the American Revolutionary War. The fort was occupied by Continental Army forces from New York and Massachusetts under the command of Colonel Peter Gansevoort. The besieging force was composed of British regulars, American Loyalists, Hessian soldiers from Hesse-Hanau, and Indians, under the command of British Brigadier General Barry St. Leger and the Iroquois leader Joseph Brant. St. Leger's expedition was a diversion in support of General John Burgoyne's campaign to gain control of the Hudson River Valley to the east.

One attempt at relief was thwarted early in the siege when a force of New York militia under Nicholas Herkimer was stopped in the August 6 Battle of Oriskany by a detachment of St. Leger's forces. While that battle did not involve the fort's garrison, some of its occupants sortied and raided the nearly empty Indian and Loyalist camps, killing some Seneca, which was a blow to the morale of St. Leger's Indian support. The siege was finally broken when American reinforcements under the command of Benedict Arnold neared, and Arnold used a ruse, with the assistance of Herkimer's relative Hon Yost Schuyler, to convince the besiegers that a much larger force was arriving. This misinformation, combined with the departure of Indian fighters not interested in siege warfare and upset over their losses from the raids, led St. Leger to abandon the effort and retreat.

St. Leger's failure to advance on Albany contributed to Burgoyne's surrender following the Battles of Saratoga in October 1777. Although St. Leger reached Fort Ticonderoga in late September, he was too late to aid Burgoyne.

The first official US flag was flown during battle on August 3, 1777, at Fort Schuyler. The Continental Congress adopted the following resolution on June 14, 1777: "Resolved, that the flag of the United States be thirteen stripes, alternate red and white; that the union be thirteen stars, white, on a blue field, representing a new constellation." There was a delay in displaying this flag. The resolution was not signed by the secretary of the Congress until September 3, though it was previously printed in the newspapers. Massachusetts reinforcements to Fort Schuyler brought news of the adoption by Congress of the official flag. Soldiers cut up their shirts to make the white stripes; scarlet material was secured from red flannel petticoats of officers' wives, while material for the blue union was secured from Capt. Abraham Swartwout's blue cloth coat. A voucher shows that Congress paid Capt. Swartwout for his coat for the flag.