Volume 12 of the Revolutionary War Series documents Washington's unsuccessful efforts to capitalize on the American victory at Saratoga and his decision to encamp the Continental army for the winter at Valley Forge. The volume opens with the British forces at Philadelphia, where they had returned following the Battle of Germantown, and the Continental army, in Washington's words, "hovering round them, to distress and retard their operations as much as possible." Recognizing the importance of restricting communication between General William Howe and the British fleet, Washington dispatched a brigade to New Jersey to assist in the defense of Forts Mifflin and Mercer, key components in the American effort to obstruct the Delaware River.

Upon receiving news of the surrender of British general John Burgoyne's army to Major General Horatio Gates at Saratoga, Washington called a council of war to consider his army's options. Although his generals advised against an immediate assault on Philadelphia, Washington perceived an opportunity to defeat Howe and dispatched his aide-de-camp Alexander Hamilton to the northern department to urge upon General Gates the "absolute necessity" of sending a "very considerable" reinforcement to the main army. If those troops arrived before the British could open a supply route on the Delaware or be reinforced from New York, then the American forces could "in all probability reduce Genl Howe to the same situation in which Genl Burgoine now is." There was little further that Washington could do to strengthen the Delaware River defenses, however, and despite the determined efforts of Fort Mifflin's defenders, the Americans were forced to evacuate the fort in mid-November following a sustained bombardment from British land and naval artillery. Moreover, British and Hessian troops from New York arrived before Washington's reinforcement and joined in the British occupation of Fort Mercer a few days later.

After the fall of the Delaware River forts, Washington and his generals began extensive deliberations about the related questions of a possible winter campaign and where to quarter the troops for the winter. The generals were nearly unanimous that a winter campaign was not feasible, but they were divided between quartering the troops at Wilmington, Delaware, or in Pennsylvania along a line from Bethlehem to Lancaster. Washington settled on the third option discussed: hutting in the Great Valley of Pennsylvania. Consequently, the volume closes in December with Washington establishing his headquarters at Valley Forge, about twenty miles northwest of Philadelphia. Valley Forge provided the army with an adequate defensive position to guard against a British surprise attack, the ability to limit British depredations in Pennsylvania, and a base to cover Lancaster and York, where the Pennsylvania state government and the Continental Congress, respectively, had moved after the evacuation of Philadelphia.

Other subjects arising in the correspondence include Thomas Conway's reputedly disparaging letter to Gates about Washington; a variety of army reforms embracing reorganization of the cavalry, the establishment of a maréchaussée, or provost corps, and the improvement of the lot of the officers and enlisted men; and a purported British peace proposal. Private correspondence discusses Mount Vernon and Washington's other landholdings.