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He deserted his unit at a Missouri River post on February 21, 1866 and subsequently reenlisted in Company H of George Armstrong Custer's Seventh Cavalry in December 1866 using the alias Frank May.
Illingworth and the U.S. 7th Cavalry Regiment departed for the Black Hills on 2 July 1874 from Fort Abraham Lincoln on the west bank of the Missouri River, seven miles south of what is now Mandan, North Dakota, and returned 30 August 1874.
It was near Porcupine Butte, on December 28, 1890 that Spotted Elks's band of Miniconjou, Lakota and members of the Hunkpapa Lakota band who joined them on their way to join Red Cloud at Pine Ridge, were intercepted by a detachment of the U.S. 7th Cavalry Regiment.