During the Spanish-American war he served with his regiment in Cuba throughout the Santiago Campaign, participating in the battle of El Caney and San Juan, July 1-3, 1898, and in the subsequent operations culminating in the surrender of Santiago. He was in the Philippine Islands during the Insurrection, taking part in a number of engagements with the insurgents in 1899 and 1900, and later served two more tours of duty in the Philippine Islands. In addition to duty with troops at various stations, in the United States, he was professor of military science and tactics at the New Hampshire college of agriculture and mechanical arts, and at the Oregon agriculture college. He also served in Alaska, Hawaii and in France and Belgium during World War 1. While serving in France he commanded the 365th Infantry, the 58th Infantry Brigade and the 182nd Infantry brigade. He returned to the United States on April 1, 1919, and at his retirement became a student officer at the army service schools, in Fort Leavenworth, Kansas. After his retirement, he was recalled to active duty and served from May 13, 1921 to June 30, 1922, as professor of military science and tactics, Montana State college of agriculture and mechanical arts. General Caldwell was awarded the Silver Star Citation by the War Department for gallantry in action during the attack against the Spanish forces at El Caney, Cuba, July 1, 1898. He was also awarded the Spanish Campaign medal, the Philippine Campaign medal, the Cuban Occupation Campaign medal, the Indian Wars Campaign medal, the World War 1 Victory medal with Aisne-Marne, St. Mihiel, Meuse-Argonne and Defensive Sector battle clasps, the Belgian Croix de Guerre and the Italian Croce de Guerra.
During the Spanish-American war he served with his regiment in Cuba throughout the Santiago Campaign, participating in the battle of El Caney and San Juan, July 1-3, 1898, and in the subsequent operations culminating in the surrender of Santiago. He was in the Philippine Islands during the Insurrection, taking part in a number of engagements with the insurgents in 1899 and 1900, and later served two more tours of duty in the Philippine Islands. In addition to duty with troops at various stations, in the United States, he was professor of military science and tactics at the New Hampshire college of agriculture and mechanical arts, and at the Oregon agriculture college. He also served in Alaska, Hawaii and in France and Belgium during World War 1. While serving in France he commanded the 365th Infantry, the 58th Infantry Brigade and the 182nd Infantry brigade. He returned to the United States on April 1, 1919, and at his retirement became a student officer at the army service schools, in Fort Leavenworth, Kansas. After his retirement, he was recalled to active duty and served from May 13, 1921 to June 30, 1922, as professor of military science and tactics, Montana State college of agriculture and mechanical arts. General Caldwell was awarded the Silver Star Citation by the War Department for gallantry in action during the attack against the Spanish forces at El Caney, Cuba, July 1, 1898. He was also awarded the Spanish Campaign medal, the Philippine Campaign medal, the Cuban Occupation Campaign medal, the Indian Wars Campaign medal, the World War 1 Victory medal with Aisne-Marne, St. Mihiel, Meuse-Argonne and Defensive Sector battle clasps, the Belgian Croix de Guerre and the Italian Croce de Guerra.