When Montgomery finally began the siege of Fort Saint-Jean, he ordered Allen and about 30 Americans to join with Livingston's Canadians to secure the south bank of the St. Lawrence River against attempts by Carleton in Montreal to relieve the siege.
 He also ordered a larger force under Brown's command to secure the area north of the fort, and to cover the road between Saint-Jean and Montreal.

Allen traveled along the southeastern banks of the Richelieu River, up to Sorel, where he crossed that river and continued up the southern shore of the St. Lawrence to Longueuil.
According to Allen's account, he met Brown there, and the two of them then hatched a plan to attack Montreal.
Brown would cross the river with 200 men at La Prairie, upriver from Montreal, and Allen, with his Americans and 80 Canadians under the command of Loiseau and Duggan, two of Livingston's captains, would cross the river at Longueuil, below the city, and the two forces would, after a prearranged signal, converge on the city itself.
When Montgomery finally began the siege of Fort Saint-Jean, he ordered Allen and about 30 Americans to join with Livingston's Canadians to secure the south bank of the St. Lawrence River against attempts by Carleton in Montreal to relieve the siege.  He also ordered a larger force under Brown's command to secure the area north of the fort, and to cover the road between Saint-Jean and Montreal.
Allen traveled along the southeastern banks of the Richelieu River, up to Sorel, where he crossed that river and continued up the southern shore of the St. Lawrence to Longueuil. According to Allen's account, he met Brown there, and the two of them then hatched a plan to attack Montreal. Brown would cross the river with 200 men at La Prairie, upriver from Montreal, and Allen, with his Americans and 80 Canadians under the command of Loiseau and Duggan, two of Livingston's captains, would cross the river at Longueuil, below the city, and the two forces would, after a prearranged signal, converge on the city itself.
Battle of Longue-Pointe