Arnold was originally supposed to take up a career in trade, but in 1757 he joined the provincial militia to fight against the French in the French and Indian Wars. He only served 13 days, though the reasons for him leaving are unclear. He became a pharmacist and bookseller in New Haven and prospered enough to establish himself in the West Indies trade. His business took a major hit with the Sugar Act of 1764 and the Stamp Act of 1765, which led him to join the Sons of Liberty.
After war broke out, he joined the Connecticut militia as a captain, assisting in the seige of Boston before suggesting they try to capture the poorly defended Fort Ticonderoga in New York. After receiving a colonel’s commission in 1775, he helped capture the fort, then carried out a raid on a fort on the Richelieu River. He next set his sights on Quebec, urging the Continental Congress to authorize an invasion. When they did so, they completely over looked him for command of the expedition. He retaliated by going directly to Washington and suggesting a second expedition, which did not go well, though it did get Arnold a promotion to brigadier general.
Arnold’s relationship with the Continental Congress was…tense, to say the least. He was frequently ignored and passed over for promotions. This made him particularly vulnerable to the manipulations of loyalists like his second wife, Peggy Shippen, whom he met while serving as military governor of Philadelphia. Arnold started providing valuable information to the British by July 1779, resulting in a court martial (he was cleared) and a personal rebuke from Washington. After Congress demanded Arnold pay 1,000 for expenses during the Quebec invasion he couldn’t document, Arnold resigned his command of Philadelphia. He took command of West Point, which he planned to hand over to the British, in August 1780. His plot to give up the fort was exposed and Arnold fled, switching sides and joining the British army.
Apparently, a soldier who betrays his own side was just what the British were looking for in the late 18th century: they appointed Arnold a brigadier general and sent him right back into the field. In the colonies. They must have been really sure he was on their side. Or they were hoping he’d get captured and hanged as a spy by his former comrades. He was not, and the British commanders turned out to be just as inclined to ignore him as the Continental Congress. Still, Arnold stuck with them, and after the war he and his family set sail for England.
In England, Arnold failed to receive permission to go out on active service (why might that be?), so he returned to his business roots. He failed to prosper and was imprisoned and nearly hanged by the French for privateering during the French Revolution. His health began to decline after the turn of the century and he died on June 14, 1801. Although he was an able military commander, he’s mostly remembered now as a vile traitor. Hilariously, the plaque on the house in central London where he lived identifies him as an “American Patriot.”