In April 1775, British troops and American militia clashed at Lexington and Concord, Massachusetts. In early May, Ethan Allen led a large militia force, the "Green Mountain Boys," in an attack on the British forts of Ticonderoga and Crown Point, near Lake Champlain. Since war had not been declared, Ethan Allen's militia surprised the tiny British garrisons (Ticonderoga: 30 soldiers, Crown Point: 10 soldiers) and quickly occupied the two forts. The Green Mountain Boys then advanced up the Richelieu River and seized Fort St. Jean.
A volunteer force of Montreal militia reoccupied Fort St. Jean and forced Ethan Allen's troops to retreat on 10 June 1775. The Green Mountain Boys, however, advanced into the St. Lawrence Valley in September. At Longue Pointe, south of Montreal, a combined force of British regulars and Canadian militia trounced the Americans. They captured Allen and most of his force. The first American attack on Canada was a dismal failure.