[4] White resigned his post in Customs when he received a commission as colonel of the 37th Illinois Volunteer Infantry Regiment on September 19, 1861.

At the Battle of Pea Ridge on March 7, 1862, his brigade of two Illinois regiments blunted the attack of Louis Hebert's Confederates.

White was promoted brigadier general on June 9, 1862, and led the "Railroad" Brigade, VIII Corps.

[5] During the Second Battle of Bull Run, this unit was posted in Martinsburg, West Virginia, on the Baltimore & Ohio Railroad.

In the face of Robert E. Lee's invasion of Maryland, White retreated into Harper's Ferry, West Virginia, and joined Colonel Dixon S. Miles and his large garrison there.

"[6] This was an unfortunate decision for White because Miles proved to be incapable of mounting an effective defense of the position.

The Battle of Harpers Ferry was fought September 12–15, 1862, from a highly disadvantageous position compounded by Miles's numerous strategic mistakes.