Final answer:
George was certain the mob would capture the fortress because of the failure of government and law, the presence of an overpowering mob spirit, and the overwhelming military engagement they faced.
Step-by-step explanation:
Based on the provided text passages, George was sure the mob would capture the fortress due to the dire circumstances described. The text makes reference to an overarching sense of futility and inevitability that hung over the characters involved. Government and law enforcement had evidently failed, mob law ruled with 'fiendishness and barbarity,' and there was no differentiation between the innocent and the guilty within George's race. A despaired outlook had already spread among the defenders, as exemplified by the mentioning of the doomed Confederacy and a capture threatening the fall of Richmond.
Furthermore, the text mentions military engagements with aeroplanes, making the capture of the fortress seem even more inevitable due to overwhelming technological advantages. Despite the disparities in power, a great conviction in their cause persisted among the defenders, as highlighted by the dedication of George's associates who were willing to give their lives for their belief. This further underscores the certainty of capture, which George viewed as an unavoidable consequence of his times.