On 17 June 1898, Krueger, along with many of his fellow high-school students, enlisted for service in the Spanish–American War with the 2nd Volunteer Infantry. He reached Santiago de Cuba a few weeks after the Battle of San Juan Hill, and spent eight months there on occupation duties, rising to the rank of sergeant. Mustered out of the volunteers in February 1899, he returned home to Ohio, planning to become a civil engineer.[3][4]