He served in Kentucky, was promoted to the rank of brigadier-general of volunteers early in 1862; took part in the second day's fighting at the battle of Shiloh, served as chief of staff under Rosecrans in the Army of the Cumberland in 1863, fought at Chickamauga, and was made a major-general of volunteers for gallantry in that battle.
Rosecrans, who on the 3/4 of October 1862 was fiercely attacked here by General Earl von Dorn, whom he repulsed, both sides suffering considerable losses in killed and wounded, and the Confederates leaving many prisoners behind.
But Halleck soon went to Washington as general-in-chief, and Grant took command of his old army and of Rosecrans' Army of the Mississippi.
Two victories (Iuka and Corinth) were won in the autumn of 1862, but the credit of both fell to Rosecrans, who commanded in the field, and the nadir of Grant's military fortunes was reached when the first advance on Vicksburg, planned on an unsound basis, and complicated by a series of political intrigues (which had also caused the adoption of the original scheme), collapsed after the minor reverses of Holly Springs and Chickasaw Bayou (December 1862).
There were soldiers more accomplished, as was McClellan, more brilliant, as was Rosecrans, and more exact, as was Buell,.
A month later, an easy triumph was obtained by McClellan and Rosecrans against the Confederates of Virginia at Rich Mountain.
A desultory duel between the forces of Rosecrans and Robert E.
Sterling Price in the meanwhile had been ordered forward against Grant and Rosecrans, and Van Dorn promised his assistance.
Before the latter could come up, however, Rosecrans defeated Price at Iuka (September 19).
The Confederates, not dismayed thereby, effected their junction and moved on Corinth, which was defended by Rosecrans and 23,000 Federal troops.
Grant's other forces were split up into detachments, and when Van Dorn, boldly marching right round Rosecrans, descended upon Corinth from the north, Grant could hardly stir to help his subordinate.
Rosecrans, however, won the battle of Corinth (October 3-4), though on the evening of the 3rd he had been in a perilous position.
- Chickasaw Bayou and Fredericksburg ended the Federal initiative in the west and the east; the Army of the Cumberland under Rosecrans alone could claim a victory.
Bragg lay at Murfreesboro (see Stone River), where Rosecrans attacked him on the 31st of December 1862.
A war of manoeuvre followed, each side being reduced in turn by successive detachments sent to aid Rosecrans and Bragg in the struggle for Tennessee.
In the centre Rosecrans and Bragg spent the first six months of the year, as it were glaring at each other.
Rosecrans manoeuvred his opponent out of one position after another until Bragg was driven back into Chattanooga.
These operations were very skilfully conducted by Rosecrans and his second-in-command, Thomas, and, at a trifling cost, advanced the Union outposts to the borders of Georgia.
Burnside and the new Army of the Ohio had now cleared east Tennessee and occupied Knoxville (September 2), and meanwhile Rosecrans by a brilliant movement, in which he displayed no less daring in execution than skill in planning, once more manoeuvred Bragg out of his position and occupied Chattanooga.
Thomas's defence won him the popular title of the "Rock of Chickamauga" and enabled Rosecrans to draw off his men, but the critical position of the Army of the Cumberland in Chattanooga aroused great alarm.
In spite of his good record Rosecrans was deprived of his command.
Sheridan distinguished himself still more at the sanguinary battle of Murfreesboro (Stone river), and on the recommendation of Rosecrans was made major-general of volunteers, to date from the 31st of December 1862.
Rosecrans with Chattanooga as his objective moved from Nashville upon General Braxton Bragg, who left the winter quarters he had established at Murfreesboro and met the Union army on Stone river immediately north of Murfreesboro, on the last day of December.