Washington now had at his disposal, all of the assets that had been missing from his arsenal since the revolution had begun. He had ample men, supplies, arms and a formidable navy. Together, this assets could be directed into one massive battle plan that would give him a decisive victory that had sorely been missing from previous years.
The combined French and American plans called for multiple forces to arrive at the same location, at about the same time. Washington's combined French and American army would leave depart New York and move south by ship. Lafayette would continue his pressure against Cornwallis, forcing him to make fortifications in the small village of Yorktown. The French Navy would secure the Chesapeake Bay and prevent reinforcements from reaching Cornwallis, and prevent Cornwallis from escaping.
Before Washington could leave New York, he had to set the stage to keep British commander Clinton in place. To do this he created the impression that he was preparing a massive attack against Clinton. To do this, Washington's plans were only known to a few trusted officers and the French Naval Commander DeGrasse.
The day after receiving word for the French Admiral DeGrasse, Washington sent word to Lafayette to keep Cornwallis engaged in Yorktown. Meanwhile, Washington and Rochambeau concocted a plan that would leave some 2500 troops to guard the Hudson Highlands with the hope that these men would keep Clinton in position before he realized that the main body of American and French troops had already moved out. It was a risky move, but one that Washington had to take and it would ultimately be one of Washington's most daring moves as commander.
The plan was to establish what would appear to be a permanent camp from which Washington could attack Staten Island. Instead this camp would be a ruse. Instead, he would leave some men here and move the bulk of his command south. The secrecy of Washington's move were kept secret until Clinton received word on September 2, 1781 that it appeared Washington was moving forces south.
On August 21, the allied armies started south, first crossing the Hudson River by King's Ferry to Stony Point, then marching behind the Palisades. Once across the river they set up what appeared to be a permanent camp from where they would mount an attack against Clinton.
On September 6, Washington's allied forces began arriving at Head of Elk (now known as Elkton) where some 2000 men were sent by boat down the Elk River to Williamsburg, VA. Washington moved on to Mt. Vernon, arriving 3 days later. Here he would meet with his heads of staff and plot out the upcoming campaign in more detail. They depart on September 12 and meet up with De Grasse 5 days later on his flagship, the Ville de Paris.
Between September 18 - 21 French and American forces that had arrived at Annapolis boarded ships that would taken them south to the Williamsburg area. Their field artillery and horses continued overland. By September 26, some 16,000 combined French / American forces were concentrated at Williamsburg.