Schuyler believed himself strong enough to begin siege operations against Fort Saint-Jean. His plan to divide the American force into three parts. One part would consist of infantrymen turned sailors and marines. A second part would consist of infantrymen and the army’s artillery. Together these first two parts would establish a base south of the fort and protect the American supply line. The third part would consist of a detachment of infantry that would circle around the fort and cut the British supply line. Schuyler anticipated that additional men and guns would arrive in the days and weeks ahead, at which point he would be able to begin attacking the fort itself.
Schuyler’s army advanced from L'Île-aux-Noix on the 10th, and landed late in the day at the abandoned “upper breastwork.” Schuyler, who was unwell, gave command of the expedition to Brigadier-General Richard Montgomery. Lieutenant-Colonel Rudolphus Ritzema commanded the detachment that was to cut the fort’s supply line.
While the infantry proceeded on land, two American row galleys, the Schuyler and Hancock, proceeded downstream. Each was armed with a 12-pounder cannon and 12 swivel guns. The galleys came under fire as they neared the “lower breastwork” from British forces on land and in the river.
The British commander at Fort Saint-Jean, Major Charles Preston, had anticipated a second American advance. To watch for such a movement, he had dispatched thirty some Canadian gentlemen volunteers and Indian allies under the command of Joseph-Dominique-Emmanuel Le Moyne de Longueuil. This force travelled upstream in two bateaux armed with swivel guns, and halted when it reached the abandoned lower breastwork. There, de Longueuil landed one part of his force, while the others remained in the boats.
When de Longueuil's men saw the American row galleys, the land forces began a long-range musket fire, and the bateaux fired grape shot from the swivel guns.
According to the one of the Americans, “Our armed boats perceiving the fire on the lake, fired three twelve-pounders, one of which took the enemy' s principal batteau directly in the bow, and tore her from stem to stern; she immediately sunk, with all the men in her, amounting to thirty-five.” 
Clearly overmatched, de Longueuil ordered a retreat, and all but six of his land force embarked in the remaining bateau and headed for the fort. The retreating bateau was fired at by the galleys, but the men aboard escaped without injury. The six who remained behind were sieurs Boucherville, de La Bruere, Campion, La Madeleine, and Perthuis, and an Abenaki indian. These men occupied a small house near the lower breastwork and kept watch on American movements.
Meanwhile, Ritzema's detachment set off to make a night march around the fort. Ritzema was in front with a small vanguard. Behind him were 60 men of the 4th Connecticut, followed by 300 men of the 5th Connecticut, and finally 140 men of the 1st New York.
Ritzema had just reached the lower breastwork when the advance fell apart. The 5th Connecticut had been ambushed in the advance on September 6, and the evening gloom promised another attack. These troops panicked when they unexpectedly encountered another group of men in the woods. Soon they, along with most of Ritzema's other men, were in flight for the upper breastwork. The Americans thought that “they had been waylaid by a party of Regulars and Indians” [emphasis in original], but “not a gun had been fired, except one by a man of the detachment.” The men in the woods had been their own comrades. 
After some time, Montgomery, Ritzema, and other officers were able to reorganize the men and put them back on the march. However, they had advanced only about 1/4 mile when the Connecticutians panicked a second time after some random shells from Fort Saint-Jean burst in the woods.
After this second retreat, Ritzema was left with less than half of his original force. Ritzema resolutely pressed on, and his men struggled to keep up in the dark, swampy woods. Near midnight, Ritzema, now with only about 50 men, at last reached the lower breastwork. There, the Americans observed a fire had been lit in a small house, and they moved to surround it. Sieurs Boucherville and La Madeleine, who were outside the house, gave the alarm and fled. When the men inside the house ran out the door, they were met with a hail of gunfire. Sieur Perthuis and the Abenaki were killed, Sieur de La Bruere was shot in both arms (but escaped), and Sieur Campion got away unharmed.
Realizing how few men were still with him, Ritzema halted and waited for stragglers to appear. Meanwhile, his men stripped and scalped Sieur Perthuis and beheaded the dead Abenaki.  Sometime before 3am, Montgomery cancelled the operation and ordered Ritzema's men back to the upper breastwork.
The next morning [September 11] the senior officers announced, in a council of war, that they favored continuing operations against Saint-Jean. However, word then came that the enemy was on the move. According to one officer: “we saw their armed schooner [the Royal Savage], of one hundred and eighty tons, carrying twelve nine-pounders, coming towards us.” It was a critical moment: the American flotilla was no match for such a vessel. 
According to Ritzema, the New York troops “remained in their Ranks & shewed a ready Spirit to proceed,” but the Connecticutians panicked and fled to the bateaux. Ritzema, in a rage, wounded several fleeing men with his sword, and had to be restrained by a doctor from using his pistol, too. He concluded, “This infamous conduct so much dispirited the General that he ordered the whole to embark and to proceed to Isle au Noix.” 
Once again, an American movement against Fort Saint-Jean had ended in disappointment.
Montgomery complained bitterly in a letter to his wife:
“…such a set of pusillanimous wretches never were collected. Could I, with decency, leave the army in its present situation, I would not serve an hour longer. I am much afraid the general character of the people has been too justly represented. However, there are some whose spirit I have confidence in; they are taking pains with the men, and they flatter me with hopes of prevailing on them to retrieve their characters.”
He also feared that any chance that the Canadians would rise up en masse to support the Americans was now lost:
“ We were so unfortunate as to have some Canadians witnesses of our disgrace! What they will think of the brave Bostonians , I know not! My own feelings tell me they are not likely to put confidence in such friends.”
1. I've had some difficulty discerning the exact composition of Schuyler's army on September 10. The following represents my understanding of the organization as of this writing (keep in mind that I'm working with a limited set of sources):
- 1st Connecticut: 2 companies, commanded by captains William Douglass and David Welch. Douglass commanded a row galley; it's possible that all of these men were used to man vessels in the American flotilla.
- 4th Connecticut: A part of the regiment, commanded by Major Samuel Elmore.
- 5th Connecticut: Most or all of the regiment, commanded by Colonel David Waterbury.
- 6th Connecticut: 1 company, commanded by Captain Edward Mott. These men were likely serving the American cannon and/or helping to man the vessels.
- 1st New York: A part of the regiment, commanded by Lieutenant-Colonel Rudolphus Ritzema.
- 2nd New York: A part of the regiment. The officers that I have been able to place with the army at this time are Captain Christopher Yates, Captain Joseph McCracken, and Lieutenant Cornelius Van Slyck.
2. I haven’t been able to find any confirmation of this disaster among Canadian sources. Although the Americans suspected that most or all of the men aboard this vessel were killed or drowned, this is may have been a case of wishful thinking. The men in the boat were thought to have been either Canadian volunteers or British regulars. At the very least, it appears from Canadian sources that no seigneurs died in his incident.
Another mystery (to me) is when exactly this incident took place. One source implies that it occurred immediately before the skirmish on land, while others imply that it took place considerably earlier.
3. Montgomery believed that the men who inadvertently triggered the panic were stragglers. Another source claimed that they were a party guarding the flank of the American column.
4. The mutilation of the dead bodies was done in retaliation for similar acts attributed to the Indians. One American claimed that after the skirmish on the 6th, “they dug up our dead and mangled them in the most shocking manner.” Perthuis may have been wearing a red coat, for he was mistaken for a British regular. An observer wrote, “We stripped the Regular and found a very fine gun and sword--the gun with two Barrels the neatest I ever saw, a fine watch some money, and very neatly dressed.”
5. The Royal Savage had only recently been launched. It was unavailable to contest the American advance on September 6. Unbeknownst to the Americans, the Royal Savage actually carried only 3, 4, and 6-pounders.
6. Ritzema was convinced that the Connecticut troops were the chief problem. Montgomery, who was also a citizen of New York, found cause for complaint with the troops as a whole.
7. This is in reference to the Canadian slang word Bostonnais, which literally means person from Boston, but was used in reference to all Americans. It carried roughly the same meaning as Yankee does today to some non-Americans.