Hill's forces from Harper's Ferry.

These attacked the enemy, drove him from the hill across the Antietam again; and so threatening did the situation at that moment appear to General McClellan, that he is said to have sent General Burnside the message: "Hold your ground! If you cannot, then the bridge, to the last man.
Always the bridge! If the bridge is lost, all is lost!" The urgency of this order sufficiently indicates that the Federal commander was not without solicitude for the safety of his own left wing.

Ignorant, doubtless, of the extremely small force which had thus repulsed General Burnside, in all four thousand five hundred men, he feared that General Lee would cross the bridge, assail his left, and that the hard-fought day might end in disaster to his own army.

That General Lee contemplated this movement, in spite of the disproportion of numbers, is intimated in his official report.

"It was nearly dark," he says, "and the Federal artillery was massed to defend the bridge, with General Porter's corps, consisting of fresh troops, behind it.
Under these circumstances," he adds, "it was deemed injudicious to push our advantage further in the face of fresh troops of the enemy much exceeding our own." The idea of an advance against the Federal left was accordingly abandoned, and a movement of Jackson's command, which Lee directed, with the view of turning the Federal right, was discontinued from the same considerations.