Following the outbreak of the Civil War, Lauman was actively engaged in recruiting volunteers to join several new military companies he was raising.

[1] His 7th Iowa was distinguished for gallant conduct during the fighting, and it suffered greater loss than any other regiment taking part in the engagement, amounting to more than 400 killed, wounded and missing.

He was appointed to lead the 4th Brigade, 2nd Division during Grant's attack on Fort Donelson in Tennessee, and was among the first troops to storm and enter the Confederate works.

In recognition of his service at Fort Donelson, he was promoted to brigadier general in the Union Army on March 21, 1862.

He was relieved of duty by the order of Maj. Gen. William T. Sherman shortly after the capture of Jackson, Mississippi, on July 16, 1863.