The event you're referring to is the Siege of Yorktown, which took place in 1781 during the American Revolutionary War. General George Washington, with the support of French forces led by General Rochambeau, successfully besieged the British army commanded by General Cornwallis in Yorktown, Virginia. The successful siege ultimately led to Cornwallis's surrender on October 19, 1781, which was a significant turning point in the war.
The defeat of Cornwallis's forces marked the last major military confrontation in the American Revolutionary War and set the stage for negotiations that eventually led to the Treaty of Paris in 1783, which formally recognized the independence of the United States. This victory was crucial in galvanizing support for the American cause and demonstrated the effectiveness of the alliance with France.