When the American Revolutionary War broke out with the Battles of Lexington and Concord in April 1775, Wooster was in charge of the militia at New Haven. Benedict Arnold wanted to march with his militia company in support of the cause, but Wooster tried to deny Arnold access to the militia's weapons stores. Arnold's threats were persuasive, and Wooster opened the magazine to him. (This confrontation is commemorated in New Haven as Powder House Day.) Wooster was first given command as a major general of Connecticut militia companies sent to defend New York City against possible British troop landings. When the Continental Army was established, Wooster was commissioned a brigadier general, and his troops were sent to be part of General Philip Schuyler and General Richard Montgomery's 1775 invasion of Quebec. Wooster participated in the Siege of Fort St. Jean in the fall of 1775, and was then given military command of Montreal after that city fell in November. He assumed command of all the forces in Quebec following the death of Montgomery in the Battle of Quebec at the end of December 1775.