Benedict Arnold is most famous for spying on the U.S. Continental Army, of which he was a general, for the British. He plotted the siege of West Point, and when his plans were discovered, he left the Continental Army and fought for the British.
Before he was the general of the Continental Army, Arnold was a member of the Sons of Liberty, and he climbed the ranks from there. He had many great successes during the Revolutionary War, but one day upon returning home, he learned that his first wife had died. Later, he met Peggy Shippen, who was the daughter of a British loyalist, and the two were married.
Through his new wife's connections, Arnold began providing the British with troop locations of the American fighters and kept them informed of where the supply depots were located. Part of the plan for the siege of West Point was a deliberate refusal to fix needed repairs to the facility. The plan was to leave the area, which was under Arnold's charge, vulnerable to attack. The plans for the siege were all laid out on paper, and when they reached President Washington, there was a warrant put out for Arnold to be kidnapped.
Arnold eventually escaped back to England where he found it difficult to join their military or find other gainful employment.