Benedict Arnold (14 January 1741 [O.S. 3 January 1740] – June 14, 1801) was an American-born military officer who served during the Revolutionary War. He fought with distinction for the American Continental Army and rose to the rank of major general before defecting to the British side of the conflict in 1780. General George Washington had given him his fullest trust and had placed him in command of West Point in New York. Arnold was planning to surrender the fort there to British forces, but the plot was discovered in September 1780, whereupon he fled to the British lines. In the later part of the conflict, Arnold was commissioned as a brigadier general in the British Army, and placed in command of the American Legion. He led the British army in battle against the soldiers whom he had once commanded, after which his name became, and has remained, synonymous with treason and betrayal in the United States.

Arnold was born in Connecticut. In 1775, when the war began, he was a merchant operating ships in the Atlantic Ocean. He joined the growing American army outside of Boston, and distinguished himself by acts that demonstrated intelligence and bravery: In 1775, he captured Fort Ticonderoga. In 1776, he deployed defensive and delay tactics at the Battle of Valcour Island in Lake Champlain that gave American forces time to prepare New York's defenses. His performance in the Battle of Ridgefield in Connecticut prompted his promotion to major general. He performed operations that provided the Americans with relief during the Siege of Fort Stanwix, and key actions during the pivotal 1777 Battles of Saratoga, in which he sustained leg injuries that put him out of combat career for several years.

Arnold repeatedly claimed that he was being passed over for promotion by the Continental Congress, and that other officers were being given credit for some of his accomplishments. Some among those in his military and political circles charged him with corruption and other bad acts. After formal inquiries, he was usually acquitted, but Congress investigated his finances and determined that he was indebted to Congress, and that he had borrowed money heavily to maintain a lavish lifestyle.

Arnold mingled with Loyalist sympathizers in Philadelphia and married into a Loyalist family when he wedded Peggy Shippen. She was a close friend of British major John André and kept in contact with him when he became head of the British espionage system in New York. Many historians see her as having facilitated Arnold's plans to switch sides; he opened secret negotiations with her friend André, and she relayed their messages to each other. The British promised £20,000 for the capture of West Point, a major American stronghold; Washington greatly admired Arnold and gave him command of that fort in July 1780. His scheme was to surrender the fort to the British, but it was exposed in September 1780 when revolution militia captured André carrying papers which revealed the plot. Arnold escaped; André was hanged.

Arnold received a commission as a brigadier general in the British Army, an annual pension of £360, and a lump sum of over £6,000. He led British forces in the Raid of Richmond and nearby areas, and they burned much of New London, Connecticut, to the ground and slaughtered surrendering forces after the Battle of Groton Heights—just a few miles downriver from the town where he had grown up. In the winter of 1782, he and Peggy moved to London, England. He was well received by King George III and the Tories but frowned upon by the Whigs and most Army officers. In 1787, he moved to Canada to run a merchant business with his sons Richard and Henry. He was extremely unpopular there and returned to London permanently in 1791, where he died ten years later.