Hallowell's fervent abolitionism led him to volunteer for service in the Civil War, and he inspired Holmes to do the same. He was commissioned a first lieutenant on July 10, 1861, joining the
20th Massachusetts Volunteer Infantry with Holmes. Hallowell fought in the
Battle of Ball's Bluff on October 21, 1861, in which he distinguished himself by leading a line of skirmishers to hold off Confederate forces. Hallowell then swam across the
Potomac River, constructed a makeshift raft, and made several trips to the Virginia bank to rescue trapped Union soldiers before his raft fell apart. Hallowell was promoted to captain on November 26, 1861. He was wounded in the
Battle of Glendale on June 30, 1862, and suffered more severe wounds in the
Battle of Antietam on September 17. His left arm was shattered by a bullet but later saved by a surgeon; Holmes was shot in the neck. Both took refuge in a farmhouse (a historic site now known as the
Royer–Nicodemus House and Farm) and were eventually evacuated. On April 17, 1863, he was promoted to lieutenant colonel, as second-in-command (after Colonel
Robert Gould Shaw) of the
54th Massachusetts, one of the first all-black regiments in the U.S. On May 30, he accepted Governor
John A. Andrew's personal request that he be made colonel in command of the
55th Massachusetts, another all-black regiment. He and his regiment were stationed at
Charleston Harbor and participated in the
siege and eventual taking of
Fort Wagner; Hallowell was one of the first to enter the fort after its abandonment. ==Personal life==