Final answer:
The French navy blockaded Cornwallis at the Battle of Yorktown and prevented his escape, leading to his eventual surrender. This was a crucial moment in the American Revolution.
Step-by-step explanation:
The French navy played a crucial role at the Battle of Yorktown by blockading Cornwallis and preventing his escape. Their fleet, led by Admiral de Grasse, sailed into Chesapeake Bay and controlled the bay, which effectively cut off Cornwallis from receiving any reinforcements or supplies. This blockade forced Cornwallis to surrender to the combined American and French forces on October 19, 1781, marking a major turning point in the American Revolution.