Thanks Prophet2520.
The American victory at Yorktown would not have been possible except for the assistance of the French fleet commanded by Admiral de Grasse at the Battle of the Virginia Capes.
Comte de Grasse prevented a British fleet from reinforcing and rescuing Cornwallis and his army allowing Washington and Continental Army to besiege and capture them.
I had an ancestor at the siege of Yorktown, a militia colonel in the blocking force across the York at Gloucester Point. Cornwallis planned to attempt an escape through Gloucester but a violent storm arose and prevented him from moving his army. And so he surrendered.