The Civil War: A Narrative: Volume One: Fort Sumter to Perryville by Shelby Foote:
It was well conceived, well thought out: McClellan took pride in the foresight and coolness which had enabled him to improvise the details under pressure. He did not consider the movement a retreat. It was a readjustment, a change of base required by a change in conditions. However, once the conference was over and the corps commanders had gone out into the night with their instructions for tomorrow, he began to consider the adverse reaction that might follow: not among his soldiers—they would understand—but among the members of the body politic, the public at large, and especially the molders of popular opinion: the editors, and later the historians. The record would speak for itself in time. He was confident that it would show how Lincoln and Stanton had thwarted him, diverting his troops when his back was turned and ignoring his pleas for reinforcements, in spite of documentary evidence that he was facing an army twice the size of his own. Meanwhile, though, he was not only in danger of being condemned and ridiculed; about to undertake one of the most difficult maneuvers in the art of war, the transfer of an army from one base to another across a fighting front, he was in danger of being physically destroyed. In that event, the record would indeed have to speak for itself, since he would not be there to supplement it before the bar of judgment. Therefore it had better be supplemented in advance, bolstered so as to present the strongest possible case in the strongest possible language. Shortly after midnight, before retiring to sleep for what he knew would be a grinding day tomorrow, he got off a wire to Stanton.
“I now know the full history of the day,” it began. After saying flatly, “I have lost this battle because my force was too small,” he got down to cases: “I again repeat that I am not responsible for this, and I say it with the earnestness of a general who feels in his heart the loss of every brave man who has been needlessly sacrificed today…. If, at this instant, I could dispose of 10,000 fresh men, I could gain a victory tomorrow. I know that a few thousand more men would have changed this battle from a defeat to a victory. As it is, the Government must not and cannot hold me responsible for the result.” The clincher came at the end: “I feel too earnestly tonight. I have seen too many dead and wounded comrades to feel otherwise than that the Government has not sustained this army. If you do not do so now the game is lost. If I save this army now, I tell you plainly that I owe no thanks to you or to any other persons in Washington. You have done your best to sacrifice this army.”
Having thus unburdened his troubled mind, and bolstered the record in the process, he took to his bed. “Of course they will never forgive me for that,” he subsequently told his wife. “I knew it when I wrote it; but as I thought it possible that it might be the last I ever wrote, it seemed better to have it exactly true.”