In 1897, the D.C. National Guard's commander, Albert Ordway, died after an extended illness. Harries was selected to replace him and was promoted to brigadier general. In April 1898, troops of the D.C. National Guard were ordered to active federal service for the Spanish–American War. They were organized as the 1st District of Columbia Volunteer Infantry, and Harries received a federal appointment to command them as a colonel. the 1st D.C. Infantry completed training in Chickamauga, Georgia. Harries then commanded the regiment in Cuba, including participation in the Siege of Santiago and the post-war Army of Occupation.