I believe he was Conscripted into the army and left at first opportunity, which appears to have been in December, 1863.

E. Stroup, Private, Company D, 18th Battalion Mississippi Cavalry,* enlisted September 1, 1863 at Tippah, Miss. by Capt. Smith for the war, recorded on the May 1 to December 31, 1864 muster roll as having deserted at Panola, Miss. May 1, 1864,** never paid, surrendered at La Grange, Tenn. May 26, 1865 and recorded as having left his command at Talahachie December, 1863

* This company subsequently became Company D, 18th Regiment Mississippi Cavalry

** Believe this is a Clerical error as all Company D records from March 1863 through April 1864 were apparently lost as they are not found in the Compiled Service Records

M269 Compiled Service Records of Confederate Soldiers Who Served in Organizations from the State of Mississippi

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Company D -- Smith Rangers (raised in Tippah County, MS)(formerly Company I, 1st Mississippi Partisan Rangers)
http://www.americancivilwarforum.com/18th-battalion-mississippi-cavalry-2450925.html