The event you are referring to is likely the Siege of Yorktown, which took place from September 28 to October 19, 1781, during the American Revolutionary War. This decisive victory by the American Continental Army, led by General George Washington, with significant assistance from French forces under General Rochambeau, effectively ended major combat operations in the war.
After a prolonged siege, British General Cornwallis was forced to surrender approximately 8,000 troops at Yorktown. This defeat significantly weakened British morale and their capacity to continue the war in America. The victory at Yorktown played a crucial role in convincing European powers, particularly France, to formally recognize American independence and provide military support, leading to increased financial aid and troop reinforcements for the Continental Army in subsequent efforts.
The surrender at Yorktown ultimately set the stage for negotiations that would result in the Treaty of Paris in 1783, formally recognizing the independence of the United States.