William Swinton, Campaigns of the Army of the Potomac

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[64] organized into brigades of four regiments each, and the brigades had been somewhat disciplined and instructed, formed divisions of three brigades each.¹ But, in armies of above sixty thousand men, it has been common, since the time of Napoleon, to create from the assemblage of two or more divisions the higher unit of the corps d'armee. As a theoretical principle of organization, General McClellan was in favor of the formation of corps; but he wished to defer its practical application until his division commanders should, by actual experience in the field, acquire the requisite training to fit them for commands so important, and until he should have learned who of his divisional officers merited this high trust.² There was much to justify this course, for there are few men able to command a body of thirty thousand men;³ and it is worthy of note that it was not till the Army of Northern Virginia had seen eighteen months of service that those at the head of military affairs in Richmond organized corps.⁴ This hesitation, however, proved unfortunate for McClellan himself; for, several months afterwards, and just as he was about moving to the Peninsula, the President divided the Army of the Potomac into four corps, and assigned to their command men whom General McClellan would not have chosen; whereas, had he created corps at first, he might have made his own selection.⁵

It next became necessary to create adequate artillery and engineer establishments, to organize the cavalry arm, and to