Arkansas Civil War Service Records of Union Soldiers, 1861-1865 is a free collection at FamilySearch.
This collection has a searchable name index for more than 100,000 records.
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Soldier’s full name
Military unit the soldier served in
Miscellaneous entries including: muster rolls, returns, rosters, payrolls, appointment books, hospital registers, prison registers and rolls, parole rolls, and inspection rolls