Shiba was then sent as a military observer to the United States and was introduced to United States Secretary of War Russell A. Alger, who agreed to having Shiba embedded within the US 5th Army Corps commanded by Major General William Rufus Shafter during the Spanish–American War. After inspecting training camps in Chattanooga, Tennessee, he landed in Cuba with the American Army and was present throughout the Siege of Santiago and subsequent attack on Puerto Rico. He returned to Japan in August 1899 and was promoted to lieutenant colonel the following month.