Soon it was discovered in Washington that Rosecrans was in trouble and required assistance.  The emergency was now too immediate to allow us to give this assistance by making an attack in rear of Bragg upon Mobile.  It was therefore necessary to reinforce directly, and troops were sent from every available point.

Rosecrans had very skilfully manoeuvred Bragg south of the Tennessee River, and through and beyond Chattanooga.  If he had stopped and intrenched, and made himself strong there, all would have been right and the mistake of not moving earlier partially compensated.  But he pushed on, with his forces very much scattered, until Bragg’s troops from Mississippi began to join him.  Then Bragg took the initiative.  Rosecrans had to fall back in turn, and was able to get his army together at Chickamauga, some miles south-east of Chattanooga, before the main battle was brought on.  The battle was fought on the 19th and 20th of September, and Rosecrans was badly defeated, with a heavy loss in artillery and some sixteen thousand men killed, wounded and captured.  The corps under Major-General George H. Thomas stood its ground, while Rosecrans, with Crittenden and McCook, returned to Chattanooga.  Thomas returned also, but later, and with his troops in good order.  Bragg followed and took possession of Missionary Ridge, overlooking Chattanooga.  He also occupied Lookout Mountain, west of the town, which Rosecrans had abandoned, and with it his control of the river and the river road as far back as Bridgeport.  The National troops were now strongly intrenched in Chattanooga Valley, with the Tennessee River behind them and the enemy occupying commanding heights to the east and west, with a strong line across the valley from mountain to mountain, and with Chattanooga Creek, for a large part of the way, in front of their line.

On the 29th Halleck telegraphed me the above results, and directed all the forces that could be spared from my department to be sent to Rosecrans.  Long before this dispatch was received Sherman was on his way, and McPherson was moving east with most of the garrison of Vicksburg.

A retreat at that time would have been a terrible disaster.  It would not only have been the loss of a most important strategic position to us, but it would have been attended with the loss of all the artillery still left with the Army of the Cumberland and the annihilation of that army itself, either by capture or demoralization.

All supplies for Rosecrans had to be brought from Nashville.  The railroad between this base and the army was in possession of the government up to Bridgeport, the point at which the road crosses to the south side of the Tennessee River; but Bragg, holding Lookout and Raccoon mountains west of Chattanooga, commanded the railroad, the river and the shortest and best wagon-roads, both south and north of the Tennessee, between Chattanooga and Bridgeport.  The distance between these two places is but twenty-six miles by rail, but owing to the position of Bragg, all supplies for Rosecrans had to be hauled by a circuitous route north of the river and over a mountainous country, increasing the distance to over sixty miles.