Why did general cornwallis decided to surrender at the battle of yorktown

General Cornwallis had brought 8000 British soldiers to Yorktown,they expected help from the British troops that were sent from new York, but the help never arrived because George Washington had help from the French. The French navy kept British troops from entering the York river while Washington and rochambeau took York town back. They surrounded the down and the seige cut off supples to the British. When they ran out of food and supplies , they could not continue to fight and so general cornallis surrendered.