Cornwallis stayed in Yorktown after de Grasse's fleet was sighted in the Chesapeake. He could have led his army away. Early in September, his forces outnumbered those of Lafayette. He had more than enough seasoned troops to fight his way out of the impending trap. Several of his officers urged him to do so. Another option would have been to ferry his force across the York River to Gloucester, and attempt an escape in another direction. Leaving Yorktown meant abandoning his hospitals, numerous loyalists who had sought his protection, and a small fleet of ships. Cornwallis decided to stay, trusting in his defenses, his well-stocked storehouses, and the British navy. Cornwallis knew that Clinton would try to rescue him. Probably he could not imagine the British navy failing to drive away de Grasse.

The allied forces in New Jersey headed south for Pennsylvania on August 30. The Continental troops paraded through Philadelphia on September 2, followed by the French the next day. The people of Philadelphia admired the soldierly appearance and elegant uniforms of their French allies. The Americans reached Head of Elk on September 6, and the French arrived two days later. Boarding the troops on ships and transporting them to the James River near Williamsburg took time. The embarkation was completed on September 18, and the last of the troops landed in Virginia on September 26.

Washington traveled ahead of his army. Along the way, he took advantage of an opportunity to visit his home at Mount Vernon, which he had not seen for six years. He reunited with Lafayette, and he and Rochambeau met with de Grasse. The first step toward the siege of Yorktown came on September 20, when a force of 1,000 Virginia militia and some French cavalry and marines cut the British escape route from Gloucester. On September 28, the allied army marched from Williamsburg to Yorktown. Altogether, Washington had with him 8,845 men, 3,000 of whom were militia. Rochambeau commanded 7,800 troops. 3,000 men from de Grasse's fleet joined the army. Cornwallis was heavily outnumbered, with some 8,500 soldiers and sailors entrenched in Yorktown.

The allies decided against an assault or a more gradual effort to starve Cornwallis out. De Grasse could not stay indefinitely. Instead, Rochambeau urged an aggressive siege, digging approaches and setting up gun emplacements, wearing the enemy down through a time-honored process “reducible to calculation.” Shortly after the allies arrived, Cornwallis pulled back to an inner line of defenses. This reduced the strain on his manpower but also cut the distance the besiegers would have to travel.

On the evening of October 5, George Washington ceremonially broke ground on the first allied parallel. The digging went forward quickly after that, the work being done at night to protect the sappers. On the afternoon of October 9, General Washington fired the first cannon shot of what became a devastating allied barrage on Yorktown. Henry Knox and his American gunners earned the respect of their French counterparts for their skill.

Every day the allies fired at least 3,600 rounds into Yorktown. Life for the British quickly became a continuous nightmare, with the troops forced to huddle in their trenches or basements for some measure of security. The allies pressed forward with their digging. By October 14, guns were in place just 150 yards from the British lines.

Blocking further progress were two British fortifications, redoubt Number 9, garrisoned by 120 men, and redoubt Number 10, guarded by seventy troops. The night of October 14, 400 French troops assaulted Number 9, and 400 Americans, led by Colonel Alexander Hamilton, attacked Number 10. The allied troops advanced with unloaded muskets, relying on their bayonets. After French troops launched a diversionary demonstration against another part of the British line, the attackers moved out and stormed the two redoubts. The fighting was brief but intense, and both redoubts were taken. The French lost fifteen killed and seventy-seven wounded. The Americans lost nine killed and thirty-one wounded. British casualties were higher. By morning, the two redoubts had been included in the advancing allied line. For the British in Yorktown, the end was in sight.