Benedict Arnold at West Point

After he became commander of Philadelphia in 1778, he met Margaret Shippen and married her the following year. The Arnolds squandered money on an extravagant social life among the Loyalist families of Philadelphia. Needing money, Arnold then began a 16-month treasonable correspondence with the British commander in chief, Sir Henry Clinton. As commandant of West Point, key to the Hudson River valley, Arnold agreed in 1780 to surrender the fort to the enemy in return for a royal commission in the British army and a sum of money. The capture of Clinton's envoy, Major John André, exposed the plot, and Arnold fled to the enemy.

Several factors stimulated Arnold's anger and treachery. These included the promotion of junior officers over him; charges by Pennsylvania authorities that he had violated military regulations, which culminated in his court-martial; and the need for ready money to reimburse wartime expenses and to pay for his Philadelphia extravagances. Arnold also disliked the Franco-American alliance (he distrusted Roman Catholic France and had not forgotten the French and Indian War).