General McDowell did not arrive; and this unprotected right flank--left unprotected from the fact that McDowell's presence was counted on--became the point of the Confederate attack.
The amount of blame, if any, justly attributable to General McClellan, first for his inactivity, and then for his defeat by Lee, cannot be referred to here, save in a few brief sentences.

A sort of feud seems to have arisen between himself and General Halleck, the commander-in-chief, stationed at Washington; and General Halleck then and afterward appears to have regarded McClellan as a soldier without decision or broad generalship.

And yet McClellan does not seem to have merited the censure he received.

He called persistently for reinforcements, remaining inactive meanwhile, because he estimated the Confederate army before him at two hundred thousand men, and was unwilling to assail this force, under command of soldiers like Johnston and Lee, until his own force seemed adequate to the undertaking.

Another consideration was, the Confederate position in front of the powerful earthworks of the city.