Answer by taking a quote from the following article:

Lewis Wallace (April 10, 1827 - February 15, 1905) was an American lawyer, Union general in the American Civil War, governor of the New Mexico Territory, politician, diplomat, and author from Indiana. Among his novels and biographies, Wallace is best known for his historical adventure story, Ben-Hur: A Tale of the Christ (1880), a bestselling novel that has been called "the most influential Christian book of the nineteenth century." Wallace's military career included service in the Mexican-American War and the American Civil War.

On February 4 and 5, 1862, prior to the advance against Fort Henry, Union troops under the command of Brig. Gen. Ulysses S. Grant and a flotilla of Union ironclads and timberclad gunboats under the command of Flag Officer Andrew Hull Foote made their way toward the Confederate fort along the Tennessee River in western Tennessee. Wallace's brigade, which was attached to Brig. Gen. Charles F. Smith's division, was ordered to occupy Fort Heiman, an uncompleted Confederate fort across the river from Fort Henry. Wallace's troops secured the deserted fort and watched the Union attack on Fort Henry from their hilltop position. On February 6, after more than an hour of bombardment from the Union gunboats, Confederate Brig. Gen. Lloyd Tilghman, surrendered Fort Henry to Grant.  Grant's superior, Maj. Gen. Henry W. Halleck, was concerned that Confederate reinforcements would try to retake the two forts when the Union troops moved overland toward Fort Donelson, so Wallace was left in command at Fort Henry to keep the forts secure. Displeased to have been left behind, Wallace prepared his troops to move out at a moment's notice. The order came at midnight on February 13. Wallace arrived along the Cumberland River the following day and was placed in charge of the 3rd Division. Many of the men in the division were untested reinforcements. Wallace's three brigades took up position in the center of the Union line, facing Fort Donelson.  During the fierce Confederate assault on February 15, and in Grant's absence from the battlefield, Wallace acted on his own initiative to send Cruft's brigade to reinforce the beleaguered division of Brig. Gen. John A. McClernand, despite orders from Grant to hold his position and prevent the enemy from escaping and without Grant's authority to take the offensive. With the Confederates continuing to advance, Wallace led a second brigade to the right and engaged the Confederates with infantry and artillery. Wallace's decision stopped their forward movement and was key in stabilizing a defensive line for the Union troops. After the Confederate assault had been checked, Wallace led a counterattack that regained the lost ground on the Union right. On March 21, 1862, Wallace, McClernand, and C. F. Smith were promoted to major general for their efforts. Wallace, who was age thirty-four at the time of his promotion, became the youngest major general in the Union army.

What rank was Lew Wallace?