Cornwallis moved down the Virginia Peninsula towards the Chesapeake Bay, where Clinton planned to extract part of the army for a siege of New York City. After surprising American forces at the Battle of Green Spring on July 6, 1781, Cornwallis received orders to move his troops to the port town of Yorktown and begin construction of fortifications and a naval yard, though when discovered American forces surrounded the town. Gen. Washington and his French ally Rochambeau moved their forces from New York to Virginia. The defeat of the Royal Navy by Admiral de Grasse at the Battle of the Virginia Capes ensured French dominance of the waters around Yorktown, thereby preventing Cornwallis from receiving troops or supplies and removing the possibility of evacuation. Following the two-week siege to Yorktown, Cornwallis decided to surrender. Papers for surrender were officially signed on October 19.