When the
American Civil War began in 1861, Fesler sided with the Union. On August 30, 1861, he enlisted into the Union Army at Morgantown, Indiana. Fesler was made the Captain of Company G in the
27th Indiana Infantry Regiment. Fesler commanded his company at the
First Battle of Winchester and at the
Battle of Cedar Mountain. In 1862, Fesler's Regiment was assigned to the
12th Army Corps, in which it would remain throughout 1863. Fesler then commanded his company during the
Battle of Antietam, near
Sharpsburg, Maryland, on September 17, 1862. His regiment spent the winter of 1862 to 1863 camped at Stafford Courthouse, Virginia, and on February 13, 1863, Fesler was promoted to Lieutenant Colonel of the 27th Indiana. In March 1863, the 12th Corps, of which the 27th Indiana was a part, was issued a star as its corps badge. On May 5, 1863, Fesler was wounded in action during the
Battle of Chancellorsville, in
Spotsylvania County, Virginia. He then temporarily commanded the 27th Indiana from July 2-3, 1863 during the
Battle of Gettysburg when the colonel of the regiment was placed in command of its brigade. Fesler and his regiment then took part in quelling the
New York Draft Riots later that month. In early 1864, Fesler's regiment was transferred to the new
20th Army Corps, which retained the old corps badge of the 12th. Now as part of the 20th Corps, Fesler and the 27th took part in the
Atlanta campaign, fighting at the
Battle of Resaca and at the
Battle of Kennesaw Mountain. He again commanded the regiment at the
Battle of Peachtree Creek on July 20, 1864, when the colonel was wounded. Fesler and the 27th Indiana mustered out in November 1864. == Later life ==