At what battle was the first official US flag flown ?

The Siege of Fort Stanwix (also known at the time as Fort Schuyler) began on August 2, 1777, 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.

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." 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.