A few months after the start of the American Civil War in early 1861, President Abraham Lincoln commissioned Denver a brigadier general in the volunteer army on August 14. In November 1861, he was ordered to report to Fort Scott in Kansas and in December, he assumed command of all Federal troops in Kansas. During March and April 1862 he commanded the District of Kansas until he was transferred to the District of West Tennessee. On 16 May 1862, Denver assumed command of the 3rd Brigade/5th Division under Maj. Gen.William T. Sherman in the midst of the Siege of Corinth. The very next day Denver's brigade participated in thefight for Russell's House. Though his brigade suffered no casualties in this engagement it was nonetheless one of the two brigades leading the attack.[1] On May 27 General Sherman again selected Denver's brigade to be one of the leading units in an attack against the Double Log House. Denver and Morgan L. Smith's brigade successfully stormed the log cabin turned block house. During this engagement Major General Ulysses S. Grantwas present on the battlefield and gave his approval at the handsome manner in which the troops behaved.[2]After the fall of Corinth Denver continued in command of his brigade, serving on garrison duty in Mississippi. During the early stages of the Vicksburg Campaign Denver was in command of the 1st Division, XVI Corps until his resignation from the Union Army on March 5, 1863.

After the war, Denver practiced law in Washington, D.C., and Wilmington, Ohio; he and his family lived in theRombach Place while residing in Wilmington.[3] He served as a delegate to the Democratic National Convention in 1876, 1880, and 1884. He was mentioned as a possible Democratic presidential nominee in 1876 and 1884, although his name was never offered into formal nomination.

James W Denver