[2] Having decided on a business career, as a teenager, he moved to Boston, where he worked as a clerk in the John Collamore & Company crockery store.

[4] With the outbreak of the American Civil War likely, Miles was among several young men in Boston who began to study drill and ceremony, tactics, and strategy under the tutelage of Eugene Salignac, who used the title "colonel" and claimed to be a French army veteran.

[1][5] The war started in April 1861; on September 9, 1861, Miles enlisted in the Union Army and was commissioned as a first lieutenant in the 22nd Regiment Massachusetts Volunteer Infantry, which was organized and commanded by Henry Wilson.

Following the war, two years later, on March 2, 1867, Miles was brevetted a brigadier general in the regular army in recognition of his earlier wartime actions of 1863 at Chancellorsville.

He was again brevetted, this time to the rank of major general, for his documented actions at the Battle of Spotsylvania Court House in 1864.

Six months after the end of the Civil War, on October 21, 1865, he was appointed a major general of volunteers at the young age of 26.

[6] After the war, he was commandant of Fort Monroe, Virginia at the mouth of the Hampton Roads harbor and the southern end of Chesapeake Bay and the James River, where former Confederate President, Jefferson Davis (1808–1889), was held prisoner.

In 1874–1875, he was a field commander in the force that defeated the Kiowa, Comanche, and the Southern Cheyenne along the upper Red River of the South.

Between 1876 and 1877, he participated in the campaign that scoured the Northern Plains after 5 companies of the 7th U.S. Cavalry Regiment under the command of Lt. Col. George Armstrong Custer were massacred at the Battle of the Little Big Horn in June 1876, and forced the Lakota Sioux tribe under chiefs Sitting Bull and Crazy Horse and their native allies onto designated federal Indian reservations.

In the winter of 1877, he drove his bluecoat mounted troops on a forced march across the eastern Montana Territory to intercept and stop the Nez Perce tribal band led by Chief Joseph (1840–1904), after the Nez Perce War, heading north to cross the border into British Canada.

Later while on the Yellowstone River, he developed expertise with the use of the heliograph for sending long-distance communications signals utilizing ancient principles of sunlight and mirrors, establishing a 140-mile-long (230 km) line of stations with heliographs connecting far-flung military posts of Fort Keogh and Fort Custer, in the Montana Territory in 1878.

Finally, young First Lieutenant Charles B. Gatewood (1853–1896), who had studied Apache culture and ways, succeeded in meeting with and negotiating a surrender of the war chief at a subsequent meeting arranged and held with General Miles, under the terms of which Geronimo and his few remaining followers agreed to temporarily spend two years in exile on a Florida reservation far to the east.

The exile included even the Chiricahuas Apache scouts who had worked for the army, in violation of Miles' original agreement with them.

[13] In 1888, Miles became the commander of the Army's Military Division of the Pacific and the Department of California, headquartered at The Presidio in San Francisco.

Two years later in April 1890, Miles was promoted to major general in the Regular Army and became the commander of the Military Division of the Missouri.

Miles was not directly involved in the tragic massacre, and was critical of the Army's commanding officer of the reconstituted / reorganized and ill-fated 7th U.S. Cavalry Regiment in the field, Colonel James W. Forsyth (1833–1906).

[19] When the United States entered World War I in 1917, the 77-year-old general offered to serve, but President Woodrow Wilson turned him down.

[citation needed] Miles died in 1925 at the age of 85 from a heart attack while attending the Ringling Bros. and Barnum & Bailey Circus in Washington, D.C., with his grandchildren.