On August 28, Gen. Philip Schuyler had assembled an expeditionary force of roughly 1,000 men at Fort Ticonderoga and set out for the Canadian border.
On September 6, the Patriots arrived at St. Johns Fort, which was situated about 12 miles southeast of Montreal and the St. Lawrence. He prepared to lay siege to the fortification commanded by British commander Sir Charles Preston. Schuyler's scouts exaggerated the strength of the British garrison and the American commander decided to withdraw to Isle aux Noix, about 10 miles south of St. Johns.
On September 10, Schuyler initiated an attack on the fort at St. Johns, but that ended in an embarrassing fiasco. He formed 2 columns to converge on the fort, but the 2 columns instead converged on each other in the dark, resulting in confusion and disorder. Both columns chose to retreat from the other. Schuyler returned to Ticonderoga feigning illness. The command of the Patriot army fell to Schuyler's second-in-command, Brig. Gen. Richard Montgomery.
Montgomery was enthusiastic and energetic. He exhibited a sharp contrast to Shuyler's slow and deliberate manner. In fact, back in August, as Shuyler seemingly wasted time in preparing his expedition, Montgomery became impatient with the deadening pace of his superior, and on August 28, he had set out with a contingent of the Patriot forces toward Montreal. It was Montgomery's brash move which got Shuyler moving. He quickly concluded his preparations and hastily set out to rendezvous with Montgomery at Ile aux Nois.
On September 16, Montgomery began a formal siege of the fortification at St. Johns. For whatever reason, Col. Charles Preston, who was noticeably outnumbered by the Americans, did not take advantage of 2 schooners which he had anchored in the Richelieu River near the fort. As the British prepared for the siege, the Patriots took control of the 2 vessels.
Montgomery sent Ethan Allen and Maj. John Brown northward from St. Johns with the purpose of recruiting Canadians who might be sympathetic to the Patriot Cause. They found about 300 Canadians who were willing to join the Patriots, but instead of returning to St. Johns, the impetuous Allen led them in an impromptu attack on Montreal.
On September 24, during the night, Allen led a part of the troops across the St. Lawrence River downriver from the lightly-defended Montreal. There, Allen waited Brown's troops, but they didn't appear. Rather, on the next day, some 250 Canadian militia troops sent by Gen. Sir Guy Carleton appeared and routed the American Patriots after firing a single volley. Ethan Allen and 35 of his troops were taken prisoner.
As October rolled around, rainy weather and a lack of supplies began to induce a dispiriting attitude in the American troops. To remedy the situation, Montgomery sent a contingent of roughly 50 Americans and 300 Canadians under Maj. John Brown and James Livingston to advance on another fortified structure, Chambly. Chambly was a venerable old stone fort which stood downriver from St. Johns and midway between it and Montreal. The 88 British troops under Maj. Joseph Stopford, who garrisoned Chambly, made no effort to use the cannon which they had, and after only a day and a half surrendered to the American Patriots. With the capture of Chambly, the Patriots now had additional ammunition and weapons to aid them in their siege of St. Johns. Nineteen cannon and about 6 tons of gunpowder were among the spoils taken.
On November 2, the siege of St. John's officially ended. This was the day that Preston had surrendered. He had lost over 700 men over the course of the 55-day siege. The news of St. Johns capitulation traveled northward and Carleton withdrew his forces from Montreal on November 12