Holmes Jr. was a Civil War veteran
https://www.lawbookexchange.com/One of the interesting facts about Oliver Wendell Holmes Jr. was that he was a veteran of the Civil War. He made the decision to enlist in the Union Army after the American Civil War broke out in 1861. He participated in the 20th Massachusetts Volunteer Regiment while there. At the Battles of Ball's Bluff, Antietam, and Chancellorsville, he earned the moniker "Harvard's Army" and sustained numerous wounds.
When the American Civil War broke out during Holmes' final year of college, he enlisted in the fourth battalion of the Massachusetts militia. Later, with his father's support, he was appointed a lieutenant in the 20th Regiment of the Army. Infantry Volunteers of Massachusetts. He saw a lot of combat, taking part in the Peninsula Campaign and the Wildlands, receiving wounds at Ball's Bluff, Antietam, and Chancellorsville, and going through a nearly fatal attack of dysentery. Henry Livermore Abbott, an officer in the 20th Massachusetts, was someone he really liked and grew close to. During the War, Holmes worked for VI Corps and attained the rank of lieutenant colonel but was not given a promotion to his regiment. In his place, Abbott took over the leadership of the unit before being slain.
During the Battle of Fort Stevens, Holmes is alleged to have yelled for Abraham Lincoln to take cover, although this is largely believed to be a hoax. Holmes himself acknowledged that he was unsure of the source of the alarm to Lincoln, and according to some accounts, he might not have been present on the day that Lincoln visited Fort Stevens. In appreciation for his military service, Holmes received a promotion to honorary colonel. After his three-year enlistment came to a conclusion in 1864, he went to his Boston home, and, worn out and ill, his regiment was disbanded.
