"On the outbreak of the Civil War Lucius Bierce organized and equipped two companies of marines. [Ambrose Bierce] joined one of these on 19th April, 1861, and two months later became part of the invasion force led by George McClellan in West Virginia.
On 6th April, 1862, Albert S. Johnson and Pierre T. Beauregard and 55,000 members of the Confederate Army attacked Grant's army near Shiloh Church, in Hardin, Tennessee. Taken by surprise, Grant's army suffered heavy losses. Bierce was a member of the force led by General Don Carlos Buell that forced the Confederate to retreat. Bierce was deeply shocked by what he saw at Shiloh and after the war wrote several short stories based on this experience."
"Bierce was promoted to the rank of second lieutenant in November, 1862. Two months later he fought at Murfreesboro where he saved the life of his commanding officer, Major Braden, by carrying his to safety and he had been seriously wounded in the fighting.
In February, 1862 Bierce was commissioned first lieutenant of Company C of the Ninth Indiana. He fought at Chickamuga (September, 1863) under General William Hazen. The sight of so many senior officers, including William Rosecrans, fleeing from the battlefield, deeply shocked Bierce. It is said that Bierce's idealism died that day and was replaced by cynicism. He later wrote that during the war he entered "a world of fools and rogues, blind with superstition, tormented with envy, consumed with vanity, selfish, false, cruel, cursed with illusions - frothing mad!"
Bierce served under General William Sherman during his Atlanta Campaign. At Resaca on 14th May, 1864, Bierce's close friend, Lieutenant Brayle was killed. Two weeks later his regiment suffered heavy losses when attacked by General Joseph Johnson at Pickett's Mill. Bierce was badly wounded at Kennesaw Mountain when he was shot in the head by a musket ball on 23rd June.
After being treated in hospital he returned to the front-line on 30th September, 1864. The injury caused him long-term problems for the rest of his life. He later wrote: "for many years afterward, subject to fits of fainting, sometimes without assignable immediate cause, but mostly when suffering from exposure, excitement or excessive fatigue."
Lt. Bierce caught a rebel musket-ball in the head at Kennesaw Mountain--that explains somethin' about "El Gringo Viejo".