While the British built up the Yorktown fortifications, Lafayette kept a watchful eye and sent word to General Washington.

Cornwallis seemed to be planning to stay in Yorktown for a while.
Also in mid-August, George Washington learned from Count de Rochambeau, commander of the French army in New England, that the French fleet commanded by Admiral de Grasse had left the West Indies headed for the Chesapeake Bay and could stay there only until October 15.

Before that Washington and Rochambeau had been considering an attack on Clinton in New York, but Washington had rounded up only 6,000 of the 10,000 American troops needed for that action.  Rochambeau had only 5,000 French soldiers.  Their combined forces would be enough, they thought, to take on Cornwallis in  Virginia.

When the American and French armies left New England in late August and passed by New York, Clinton thought they were practicing an elaborate deception to lure him from his stronghold.