Answer:
To recall his service in a Union cavalry unit during the war
Step-by-step explanation:
Pawnee warrior, who participated on the side of the Union during the American Civil War, otherwise known as Loots-Tow-Oots (Rattlesnake) or George Esaw, Pawnee, together with his Wife, took a photograph in 1868. In the picture, they were seen wearing Partial Native Dress, in which Pawnee is in Partial Civil War Uniform, where Loots-tow-oots is seen sitted, carrying a sword in his hand.
Hence, the reason Pawnee warrior Loots-Tow-Oots hold a sword in the photograph taken of him and his wife in 1868 is "to recall his service in a Union cavalry unit during the war."