The Union lines were shorter than those of the enemy.
Thus the enemy, with a vastly superior force, was strongly fortified to the east, south, and west, and commanded the river below.

Practically, the Army of the Cumberland was besieged.

The enemy had stopped with his cavalry north of the river the passing of a train loaded with ammunition and medical supplies.

The Union army was short of both, not having ammunition enough for a day's fighting.
General Halleck had, long before my coming into this new field, ordered parts of the 11th and 12th corps, commanded respectively by Generals Howard and Slocum, Hooker in command of the whole, from the Army of the Potomac to reinforce Rosecrans.

It would have been folly to send them to Chattanooga to help eat up the few rations left there.