Arnold began the war when he was elected as a captain in Connecticut's militia in March 1775. After the start of fighting at Lexington and Concord the following month, his company marched northeast to help at the siege of Boston that followed. Arnold told the Massachusetts Committee of Safety of his idea to seize Fort Ticonderoga in New York, which he knew was poorly defended. They made him a colonel on 3 May 1775, and he immediately rode off to the west, arriving at Castleton in the disputed New Hampshire Grants (present-day Vermont) in time to join with Ethan Allen and his men in the capture of Fort Ticonderoga. He followed up that action with a bold raid on Fort Saint-Jean on the Richelieu River north of Lake Champlain. When a Connecticut militia force arrived at Ticonderoga in June, he had a dispute with its commander over control of the fort, and resigned his Massachusetts commission. He was on his way home from Ticonderoga when he learned that his wife died earlier in June.