On August 14, 1862, Pitman left school without his family's knowledge and volunteered to serve in the Union Army and fight in the Civil War.
He apparently never informed his family in advance about the choice to join the war, because the news of his enlistment was reported back in Hawaiʻi's American missionary community as "Henry Pitman has run away from home and gone [to war]."
Carter described Pitman's rationale for enlisting: "In the midst of the clamor of war, when the very air vibrated with excitement, the wild enthusiasm of the crowds, and the inspiring sound of the drum, his Indian nature rose within him.
His resolve was made."

Pitman was a hapa-haole, of part Hawaiian and part Caucasian descent.
His father was white and his native-born mother was also part Caucasian from her own mother, who was the daughter of Captain Cox and a Hawaiian chiefess.
Despite his mixed-race ancestry, Pitman avoided the racial segregation imposed on other Native Hawaiian volunteers in this period.
Most Native Hawaiians who participated in the war were assigned to colored regiments, but Pitman's fair skin color meant he was able to serve in a white unit, indicating that unit assignment may have been influenced by how dark Hawaiians appeared.
Historians Bob Dye, James L. Haley and others claimed Pitman was placed in the colored regiments because of his mixed race, but regiment records indicate otherwise.

Pitman served as a private in the 22nd Regiment Massachusetts Volunteer Infantry, Company H. This regiment was also named the "Henry Wilson's Regiment" after Colonel and U.S. Senator Henry Wilson, who commanded the unit in 1861.
Colonel William S. Tilton was the regimental commander during Pitman's brief term of service.
The regiment was part of the V Corps of the Army of the Potomac under the senior command of Major General George B. McClellan.
During this period, the regiment fought in the Second Battle of Bull Run and was involved in the Maryland Campaign fighting in the Battle of Antietam, the bloodiest single-day battle in American history, and the Battle of Shepherdstown.
His regiment was on the march to the Battle of Fredericksburg when Pitman was captured by Confederate troops.
On August 14, 1862, Pitman left school without his family's knowledge and volunteered to serve in the Union Army and fight in the Civil War. He apparently never informed his family in advance about the choice to join the war, because the news of his enlistment was reported back in Hawaiʻi's American missionary community as "Henry Pitman has run away from home and gone [to war]." Carter described Pitman's rationale for enlisting: "In the midst of the clamor of war, when the very air vibrated with excitement, the wild enthusiasm of the crowds, and the inspiring sound of the drum, his Indian nature rose within him. His resolve was made."
Pitman was a hapa-haole, of part Hawaiian and part Caucasian descent. His father was white and his native-born mother was also part Caucasian from her own mother, who was the daughter of Captain Cox and a Hawaiian chiefess. Despite his mixed-race ancestry, Pitman avoided the racial segregation imposed on other Native Hawaiian volunteers in this period. Most Native Hawaiians who participated in the war were assigned to colored regiments, but Pitman's fair skin color meant he was able to serve in a white unit, indicating that unit assignment may have been influenced by how dark Hawaiians appeared. Historians Bob Dye, James L. Haley and others claimed Pitman was placed in the colored regiments because of his mixed race, but regiment records indicate otherwise.
Pitman served as a private in the 22nd Regiment Massachusetts Volunteer Infantry, Company H. This regiment was also named the "Henry Wilson's Regiment" after Colonel and U.S. Senator Henry Wilson, who commanded the unit in 1861. Colonel William S. Tilton was the regimental commander during Pitman's brief term of service. The regiment was part of the V Corps of the Army of the Potomac under the senior command of Major General George B. McClellan. During this period, the regiment fought in the Second Battle of Bull Run and was involved in the Maryland Campaign fighting in the Battle of Antietam, the bloodiest single-day battle in American history, and the Battle of Shepherdstown. His regiment was on the march to the Battle of Fredericksburg when Pitman was captured by Confederate troops.
Henry Hoʻolulu Pitman