Sherman had, as already stated, crossed to the north side of the Tennessee River at Brown's Ferry, in full view of Bragg's troops from Lookout Mountain, a few days before the attack.
They then disappeared behind foot hills, and did not come to the view of the troops on Missionary Ridge until they met their assault.

Bragg knew it was Sherman's troops that had crossed, and, they being so long out of view, may have supposed that they had gone up the north bank of the Tennessee River to the relief of Knoxville and that Longstreet was therefore in danger.

But the first great blunder, detaching Longstreet, cannot be accounted for in any way I know of.

If he had captured Chattanooga, East Tennessee would have fallen without a struggle.

It would have been a victory for us to have got our army away from Chattanooga safely.