Born in Massachusetts in 1839, Miles began his military career in the Union Army in the Civil War. At age 25, he was eventually put in command of the 2nd Corps of the Army. Later, in the 1870s and 1880s, he led successful campaigns against Native American tribes, and defeated such notable figures as Geronimo, Sitting Bull, and Chief Joseph of the Nez Perce tribe. In 1895, he was named Commanding General of the U.S. Army, a post he would retain through the Spanish-American War.

General Miles was especially interested in Puerto Rico, and even recommended its invasion prior to that of Cuba. He commanded forces at Cuban sites such as Siboney, and after the surrender of Santiago de Cuba by the Spaniards, he personally led the invasion of Puerto Rico, beginning in Guánica. He was the first head of the military government established on the island as well, acting as both heads of the army of occupation and administrator of civil affairs.

He achieved the rank of Lieutenant General in 1900 based on his performance in the war. Following the war, General Miles wrote several books and served on various commissions, including one that reported on the problems that arose in the military government in the Philippines.