Clinton sent a series of letters to Cornwallis that summer, changing the destination of the 3,000 men he had ordered out of Virginia. Though Clinton expressed his dissatisfaction with the state of affairs in Virginia, he also assured Cornwallis that he could keep the men if necessary. Cornwallis thought this necessary. The last of these letters told Cornwallis that he could keep the troops, but directed him to occupy the Yorktown peninsula. This Cornwallis did. It is not clear why Clinton left Cornwallis in Virginia. Probably he did not want to force a confrontation with Cornwallis that would be heard in London. The men of Cornwallis's army began to dig fortifications. As they built up their base at Yorktown, Cornwallis's redcoats became the target of the most ambitious Franco-American combined arms operation of the war.

For two years, Washington had done little in the north. Maneuvering in the vicinity of New York City led to nothing. Washington's main concern was keeping his army in being. This was no easy task. Supplies were always inadequate, and Continental currency was virtually worthless. Morale in the north was depressed by the lack of action and the news of defeats in the south. Recruits were hard to come by. In the summer of 1781, many of Washington's most reliable veterans were serving in the south.

Rochambeau's forces at Newport offered Washington the muscle he needed for offensive operations. He dreamed of a successful descent on New York City, erasing his embarrassing defeats of 1776. Rochambeau took a dim view of the prospects for such an assault. On May 21, Washington and Rochambeau met at Wethersfield, Connecticut, to discuss strategy. Washington argued for an attack on New York City. Rochambeau urged an operation against the forces of Phillips and Arnold in Virginia. In the end, Rochambeau agreed to cooperate with Washington against Clinton. He did not at first tell Washington that he had received word that Admiral de Grasse had sailed from France, bound for the West Indies. Rochambeau secretly urged the Admiral to sail to the Chesapeake.

Rochambeau marched nearly 5,000 troops to join Washington outside New York City. The two armies met near White Plains on July 6. Rochambeau had sent word to Washington that de Grasse's fleet was on its way. The presence of de Grasse's powerful fleet in American waters made possible a heavy blow against the British. Washington still hoped to take New York City, but an exploratory probe by Benjamin Lincoln against the British fortifications on the north end of Manhattan found them formidable. Between July 21 and 24, Washington and Rochambeau conducted a formal reconnaissance of the British defenses. It became obvious that Clinton's position was too strong to be taken by storm. Clinton had 14,000 troops, outnumbering the combined allied army. The British were well entrenched, and the Royal Navy guarded the waters around the city.

Though Washington's great desire was to enter New York City, the eruption of Cornwallis and his army into Virginia had made Washington amenable to an operation on the Chesapeake. Cornwallis and his army made a tempting target. Washington told Rochambeau that the Chesapeake could be a “more practicable and equally advisable” destination for de Grasse, and that the Admiral should take what course he deemed best for his fleet. Washington even began to ponder the idea of marching farther south to the Carolinas and joining with Greene and de Grasse in a descent upon Charleston.

Therefore, when word came on August 14 that de Grasse was on his way to the Chesapeake with twenty-nine ships of the line, four frigates, and 3,200 troops, Washington quickly dropped any lingering notions of a campaign around New York and decided on “an operation to the Southward.” He wrote to Lafayette, ordering him to keep Cornwallis confined within the Yorktown peninsula. A letter was sent to de Grasse, informing him that Washington and Rochambeau were marching to join him. The Americans and French began to gather transports at Head of Elk in Pennsylvania to facilitate the movement of the allied army once it reached the Chesapeake. Admiral Paul-François-Jean-Nicolas, comte de Barras, the commander of the French fleet at Newport, set off for the Chesapeake with Rochambeau's siege guns.

Washington and Rochambeau started their movement south on August 21. The allied army of 3,000 Americans and 4,000 French crossed the Hudson and moved into New Jersey. Security about the operation was very tight. Only senior officers knew the ultimate destination of the army. Concentrating forces in New Jersey might be a prelude to an attack on New York City. To persuade the British that this was the intention of the allies, the French began building ovens in their camp, a normal preliminary to a siege. Washington saw to it that false plans for an offensive against New York City were intercepted by the British.