In Feb., 1777, Congress, despite General Washington's protests, promoted five brigadier generals of junior rank to major generalships over Arnold's head. This and subsequent slights by Congress embittered Arnold and may in part have motivated his later treason. Although he soon won promotion by his spectacular defense (1777) against William Tryon in Connecticut, his seniority was not restored. In the Saratoga campaign, his relief of Fort Stanwix and his brilliant campaigning under Horatio Gates played a decisive part in the American victory. He became (1778) commander of Philadelphia, after the British evacuation, and there married Peggy Shippen, whose family had Loyalist sympathies.

In 1779 he was court-martialed because of disputes with civil authorities. He was cleared of all except minor charges and was reprimanded by Washington; nevertheless he was given (1780) command of West Point. He had already begun a treasonable correspondence with Sir Henry Clinton in New York City, and now arranged to betray West Point in exchange for a British commission and money. The plot was discovered with the capture of John André, but Arnold escaped. In 1781, in the British service, he led two savage raids—against Virginia and against New London, Conn.—before going into exile in England and Canada, where he was generally scorned and unrewarded.