[1] He volunteered to surrender his leave, joined the 9th Infantry when its train passed through Syracuse, and took part in the regiment's strike response activities.

[1] During the Spanish–American War, he served with his regiment in Cuba, and took part in the Battle of El Caney and Siege of Santiago, for which he received the Citation Star.

[1][4] Barker contracted yellow fever and returned to the United States to convalesce, after which he performed temporary recruiting duty in Norfolk, Virginia.

[1] In October 1898, he took part in one of the last conflicts of the American Indian Wars when his unit fought in the Battle of Sugar Point at Leech Lake, Minnesota.

[4] Barker served with the 3rd Infantry at Madison Barracks from April 1912 to June 1914, when he was invited to become a stagiaire (student and intern) with the French Army.

[7] When the American Expeditionary Forces arrived in France in the spring of 1917, Barker was promoted to lieutenant colonel and served on General John J. Pershing's staff.

[4] For his World War I service, Barker was a recipient of the French Legion of Honor (Officer) and Croix de guerre.