In the summer of 1781, British armies held Charleston and New York, safe so long as George Washington lacked the naval power needed for a combined land and water siege. On 2 August 1781, Lord Cornwallis chose Yorktown as his winter position, confident that the Royal Navy could rescue him if necessary. But after the Comte de Grasse informed Washington on 28 July 1781 that he would sail for the Chesapeake instead of New York, Washington read the letter on 14 August and ordered the Continental Army and Comte de Rochambeau's French forces to Virginia to trap Cornwallis, while Lafayette was told on 15 August to block any retreat through North Carolina and conceal the expected arrival of the French fleet. De Grasse had already sailed north on 5 August with 28 ships of the line from Cape Francois, and when the allied armies marched from White Plains on 18 August, the parts of the siege ring were moving into place. The French fleet anchored off Cape Henry on 29 August, and although Washington's army was still far to the north by the end of September, the 3,300 infantry de Grasse brought from the West Indies made Lafayette strong enough to prevent escape by land. Cornwallis's only remaining hope was escape by sea, but de Grasse destroyed that chance at the Battle of the Capes on 5 September, and Admiral Barras strengthened the blockade when he entered the Chesapeake with 8 ships of the line and siege artillery on 10 September. Outnumbered by the French fleet, British Admiral Thomas Graves returned to New York City, and de Grasse's decision to bring nearly all his ships to the Chesapeake, his victory in the Battle of the Capes, and Barras's timely arrival gave Washington the temporary naval superiority he had long sought. L'Hermione, which left Boston Harbor on 10 September 1781 carrying 6,000 pounds of powder for the siege artillery, joined the blockade on 28 September, the same day allied land forces completed their march from Williamsburg to Yorktown. Trapped at Yorktown by more than 18,500 allied troops and de Grasse's fleet, the British army surrendered there in 1781 through the combined effect of Washington's and Rochambeau's generalship and what was called the most perfectly executed naval campaign of the age of sail.
