Final answer:
Benedict Arnold, a patriot military leader, later became a traitor and sold military information to the British during the American Revolutionary War.
Step-by-step explanation:
The patriot military leader who later became a traitor and sold military information to the British was Benedict Arnold. He was a major during the American Revolutionary War and had initially served as a successful commander for the American forces. However, due to personal conflicts and disillusionment, Arnold switched sides and betrayed his country.