In New York, Washington learned that the French Admiral François Joseph Paul de Grasse was sailing north from the West Indies. Rochambeau convinced Washington they could attack and defeat Cornwallis. The two commanders began marching south to meet de Grasse's 27 ships and 3,200 troops. Clinton learned of this and sent a British naval fleet to the Chesapeake Bay to cut them off. But the French and Americans arrived first. In the Battle of the Chesapeake the French navy forced the British ships back to New York City. Clinton warned Cornwallis in early September he would soon be facing American and French troops but promised him reinforcements by sea. Clinton had not yet learned of the French naval victory and did not know he could not rescue Cornwallis by sea. By the end of September, Washington and Rochambeau had surrounded Yorktown. Cornwallis received another message in late September from Clinton telling him help was on the way. Cornwallis was misled again and thought he only had to hold out for a few days.