Schuyler is defending the decision not to advance further into Canada without first capturing Fort St. John. If the army had left a portion of its strength besieging St. John while the rest moved north, there was a possibility of “effectually cut[ting] off every possibility of Retreat.”
In that case, says Schuyler, “the Canadians I do firmly believe would immediately have joined Mr [Guy] Carlton,” the imperial governor of Quebec, and taken up arms against the stranded Continental army. https://founders.archives.gov/documents/Washington/03-02-02-0291