By the mid-1770s, St. Clair considered himself more of an American than a British subject. In January 1776, he accepted a commission in the Continental Army as a colonel of the 3rd Pennsylvania Regiment. He first saw service in the later days of the Quebec invasion, where he saw action in the Battle of Trois-Rivières. He was appointed a brigadier general in August 1776, and was sent by Gen. George Washington to help organize the New Jersey militia. He took part in George Washington's crossing of the Delaware River on the night of December 25–26, 1776, before the Battle of Trenton on the morning of December 26. Many biographers credit St. Clair with the strategy that led to Washington's capture of Princeton, New Jersey, on January 3, 1777. St. Clair was promoted to major general in February 1777.