When the Spanish–American War broke out in May 1898, Sumner was appointed brigadier general of volunteers and placed in command of the 1st Brigade, Cavalry Division, V Corps. He was sent to Cuba with the corps and led his brigade into action at the Battle of Las Guasimas. When Joseph Wheeler fell ill, Sumner temporarily assumed command of the Cavalry Division. He was still in command of the division when the fighting at the Battle of San Juan Hill began. However, it is said that when Wheeler heard the sound of battle he rose from his sick bed and rushed to take command of the division. Sumner returned to command his brigade for the rest of the battle and in the Siege of Santiago. In September, he was appointed major general of volunteers. He was discharged from the volunteers and reverted to his Regular Army rank of colonel on April 15, 1899.