…of the first of September, after his interview with Kutuzov, Count Rostopchin had returned to Moscow mortified and offended because he had not been invited to attend the council of war, and because Kutuzov had paid no attention to his offer to take part in the defense of the city; amazed also at the novel outlook revealed to him at the camp, which treated the tranquillity of the capital and its patriotic fervor as not merely secondary but quite irrelevant and unimportant matters.
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The sentence should be deleted because it introduces information that is irrelevant to the passage.
The writer should deleted the sentence because it provides background information that is irrelevant to the paragraph.