When the
American Civil War began in 1861, Chetlain chose to follow the
Union cause. He was said to have been the first man in
Illinois to volunteer for the Civil War. On April 16, he was part of a meeting held in Galena to raise a volunteer company and, with
Ulysses S. Grant, was prominent in its creation. When Grant declined the post, he suggested for Chetlain to be elected as the company's captain. In
Springfield in late April the
12th Illinois Infantry was organized, and Chetlain's company was added to it. On May 2 he entered the Union Army as
captain of the 12th Illinois, and the next day, he was elected the regiment's
lieutenant colonel. Shortly afterward, the 12th Illinois was ordered to
Cairo, Illinois, and that September, it was part of the force used by Grant in the expedition against and capture of
Paducah. Chetlain was then given command of
Smithland, Kentucky, where he constructed fortifications to defend part of the
Cumberland River. In January 1862, he rejoined his regiment, led it during the expedition up the
Tennessee River, was at the capture of
Fort Henry on February 6, and led his command during the
Battle of Fort Donelson from February 12 to 16. Chetlain was then given command of
Corinth, Mississippi, where he began to organize and train
black soldiers for Union Army service. On June 18, 1865, Chetlain was
brevetted to the rank of
major general. Between 1865 and 1866, he commanded at Memphis and then at
Talladega, Alabama On January 26, 1866, Chetlain was mustered out of the Union Army. His wartime performance has been described as follows: ==Later life==