Montgomery's expedition set out from Fort Ticonderoga in late August, and in mid-September began besieging Fort St. Johns, the main defensive point south of Montreal. After the fort was captured in November, Carleton abandoned Montreal, fleeing to Quebec City, and Montgomery took control of Montreal before heading for Quebec with an army much reduced in size by expiring enlistments. There he joined Arnold, who had left Cambridge in early September on an arduous trek through the wilderness that left his surviving troops starving and lacking in many supplies and equipment.