He asked to be relieved from further duty in the capacity in which he was engaged and his request was granted.

When General Halleck took the field in April, 1862, Sheridan was assigned to duty on his staff.

During the advance on Corinth a vacancy occurred in the colonelcy of the 2d Michigan cavalry.
Governor Blair, of Michigan, telegraphed General Halleck asking him to suggest the name of a professional soldier for the vacancy, saying he would appoint a good man without reference to his State.

Sheridan was named; and was so conspicuously efficient that when Corinth was reached he was assigned to command a cavalry brigade in the Army of the Mississippi.

He was in command at Booneville on the 1st of July with two small regiments, when he was attacked by a force full three times as numerous as his own.