During 1781 and 1782, Washington had been working toward a decisive conclusion of the war. As a result of reports from an intelligence service led by Benjamin Franklin in Paris, Washington was convinced that British public opinion definitely was turning against continuing the American war. Washington knew if the British army could be trapped between American land forces and the superior French fleet for a sufficient period of time the British could be compelled to surrender.
In September 1779 the fleets and armies of France and Spain attacked the British fortress of Gibraltar. Great Britain could not afford to lose its precious gateway to the Mediterranean. Because Gibraltar could be reinforced and supplied only by sea, its support became the most important responsibility of the British fleet.
In 1781, when Gibraltar was especially hard pressed, 29 French ships of the line under Adm. François Joseph Paul, comte de Grasse, were able to sail from Brest on March 22, bound initially for the West Indies but with orders to be off the American coast in July and August. Washington learned of the French fleet's departure on May 22 and with Rochambeau planned to attack Clinton in New York City. In June, French troops were recalled from Newport, R.I., to join Washington's forces. The New York offensive never materialized, however, because Clinton's forces, reinforced by an additional 3000 German troops, were too strong, and the New England militia failed to come forward in sufficient numbers.
On August 14, Washington received word that de Grasse was bringing the French fleet to Chesapeake Bay. He immediately decided to attack Cornwallis at Yorktown, Virginia. The troops of Washington and Rochambeau marched south, leaving a containing force to watch Clinton in New York. De Grasse's fleet arrived at the Chesapeake capes on August 30, drove off a British fleet under Adm. Thomas Graves and established a tight blockade of Cornwallis's army. Some 16,000 American and French troops and Virginia militia, under Washington's command, laid siege to Yorktown. Cornwallis made several vain attempts to break through allied lines, but on Oct. 19, 1781, he was obliged to surrender.