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At this point, McClernand had already pitched his new troop-raising plan to Lincoln, proposing his western men be formed into new regiments and assigned to him for the capture of Vicksburg.
McClernand, in Washington, was about to embark on a visit to the battlefield and McClellan. Wilson was to pave the way with a Western offer McClellan could not refuse. (This, again, via Longacre.)
If all this is news to you, take pleasure in Civil War history, where a long tradition of authorial negligence and irresponsibilty breeds innumerable surprises for the reader.
(Top right, McClernand and Lincoln at Antietam - ever wonder what they were doing there?)