When the Civil War erupted, McCook recruited a company and joined the 31st Illinois Infantry, serving under his friend, Col. John A. Logan.

[1] On April 16, 1867, the United States Senate confirmed McCook's nomination for appointment to the grade of brevet major general of volunteers to rank from March 13, 1865.

Wintermute declared he would get even, borrowed a pistol from a friend, and shot McCook four times in the chest at close range as he returned from washing his hands.

McCook was reportedly able to still attack Wintermute, and needed to be restrained so medical care could be provided; he died the following day from blood loss.

Wintermute was tried, convicted and sentenced to 10 years for manslaughter, but obtained a new trial and was acquitted in Vermillion[3]—due to the chaotic and confusing chain of events.