 While Cornwallis fortified , forces from the West Indies and the allied camp outside New York were preparing to converge on his position . With the arrival of the French fleet under the Comte de Grasse at the end of August , and the arrival of General Washington 's combined French @-@ American army by late September , Cornwallis was trapped . After the Royal Navy fleet under Admiral Thomas Graves was defeated by the French at the 5 September Battle of the Chesapeake , and the French siege train arrived from Newport , his position became untenable . 
