When tensions leading up the American Revolutionary War increased, Maxwell resigned his commission and became active in Patriot political and resistance activity in New Jersey. When the war broke out he was commissioned as colonel of the 2nd New Jersey Regiment in November 1775. The regiment was among troops sent to Quebec under General John Sullivan in early 1776, and was involved in the Battle of Trois-Rivières before the Continental Army retreated to Fort Ticonderoga. Promoted to brigadier general, Maxwell returned to New Jersey to join General George Washington's army after its retreat across New Jersey following the loss of New York.