The Siege of Yorktown was the 1781 victory that helped force Britain to negotiate an end to the American Revolutionary War.
American forces under George Washington and French forces commanded by Jean-Baptiste Donatien de Vimeur, comte de Rochambeau, trapped British troops led by Charles Cornwallis at Yorktown, Virginia. French naval support helped prevent a British evacuation by sea, while allied troops surrounded the position on land.
Cornwallis surrendered on October 19, 1781. The victory did not immediately end every military operation, but it made continued British prosecution of the war politically difficult. Peace negotiations followed, eventually producing the Treaty of Paris in 1783, which recognized American independence.
Saratoga was another major American victory and helped secure French support, but Yorktown is the battle most directly associated with the war's decisive final campaign. This chain from independence declaration to recognized sovereignty gives added historical context to America250.