In April 1777, Continental Army Major General Philip Schuyler ordered the 3rd New York Regiment under the command of Colonel Peter Gansevoort to occupy and rehabilitate the fort as a defense against British and Native incursions from Quebec. Arriving in May, they immediately began working on the fort's defenses. Although they officially renamed the fort to Fort Schuyler, it was still widely known by its original name.
Warnings from the friendly Oneida Indians that the British were planning an expedition to the Mohawk Valley were confirmed by mid-July, spurring the pace of the work. In early July, Gansevoort reported on the state of affairs to Schuyler, noting that provisions and ammunition were in short supply. Schuyler ordered additional supplies sent to the fort on July 8.
St. Leger, who was brevetted a brigadier general for the expedition, assembled a diverse force consisting of British regulars from the 8th and 34th Regiments, a number of artillerymen, 80 jäger from Hesse-Hanau, 350 Loyalists from the King's Royal Regiment of New York, a company of Butler's Rangers, and about 100 Canadien laborers. His artillery consisted of two six-pound pieces, two 3-pounders, and four small mortars. He expected these to be adequate for the taking of a dilapidated fort with about 60 defenders, which was the latest intelligence he had when the expedition left Lachine, near Montreal, on June 23.
St. Leger first learned that the Americans had occupied Stanwix in force when prisoners captured from its garrison were brought to him on the St. Lawrence. He learned from the prisoners that Fort Stanwix had been repaired and was 'garrisoned by upwards of 600 men ... and the rebels are expecting us, and are acquainted with our strength and route'. Daniel Claus, the Indian agent accompanying the expedition, convinced St. Leger to go to Oswego, where a body of Indians could be recruited. They arrived at Oswego, New York on July 14, where Joseph Brant and about 800 Indians joined the expedition. These consisted mainly of Mohawks and Senecas, but there were also warriors from the other tribes of the Iroquois League (other than the Oneidas and the Tuscaroras, who still claimed neutrality), and some Indians from the Great Lakes area.
After leaving Oswego another report reached St. Leger that more supplies were en route to the fort. St. Leger immediately dispatched Brant with 200 Indians and 30 regulars to intercept those supplies and begin besieging the fort. Brant's arrival at the fort on August 2 was just too late. The supply convoy, which was guarded by 200 men from the 9th Massachusetts Regiment, had arrived and been unloaded. Brant's men were able to capture the convoy's boat captain; the Massachusetts men remained in the fort.