Civil War Union Army Officer. He entered the Union Army as a Private in Company H, 19th Pennsylvania Volunteer Infantry. He served with his unit through it's three month enlistment, and was honorably mustered out on August 29, 1861. On September 11, 1861 he secured an appointed as a Captain in the newly raised 69th Pennsylvania Volunteer Infantry, and was assigned to command Company K. Over the next year and a half he and his men participated in a number of engagements, seeing action at the Battles of Fair Oaks, Gaines Mill, Malvern Hill, Antietam and Fredericksburg. At the July 1863 Battle of Gettysburg, Captain Davis' men were stationed at "The Angle" on Cemetery Ridge, and born the brunt of the Confederates who made it that far during Pickett's Charge on the Third Day of the engagement (July 3, 1863). The fighting saw the death of the 69th Pennsylvania leader, Colonel Dennis O'Kane, its second in command, Lieutenant Colonel Martin Tschudy, and the severe wounding of it's Major, Micheal Duffy. In the aftermath of the battle Captain Davis, being the highest ranking unwounded officer, took command of the survivors of the 69th. He led his regiment as it pursued the Confederate army back to Virginia, and received a promotion to Major (dated July 4, 1863). When now Lieutenant Colonel Duffy resigned from his wounds in December 1863, William Davis became the official commander of the regiment. Leading the unit through the 1864 battles of the Wilderness, Spotsylvania Court House, Cold Harbor and the Siege of Petersburg, he received a promotion to Lieutenant Colonel on July 4, 1864. At little over a month later he was wounded in the right knee at the August 25, 1864 Battle of Ream's Station. Kept out of action for a time, when he returned he received an appointment to Colonel on January 1, 1865. However, since his regiment had been greatly reduced due to its nearly 4 years of field service, he was never mustered into that rank. He saw the war through to it's conclusion, and was honorably mustered out on July 1, 1865.