Mr. Martin -- the "bushwhackers" were almost certainly Confederate. There were three or four companies of partisan rangers operating south of the Arkansas River between Fort Smith and Dardanelle in 1864-1865. They raided Union supply points, and ambushed Union wagon trains and patrols, and were pretty effective in tying down Federal troops. They were considered to be bushwhackers, robbers and murderers by the Yankees -- Note: you can usually judge the effectiveness of Confederate partisan rangers by the vehemence with which the Yankees denounced them. The more effective the partisan rangers were, the uglier the names the Yankees called them.
In addition to the activities of these Confederate troops, are isolated reports bands of unaffiliated opportunists who preyed on civilians in the area, but nothing on the scale of the state of anarchy that existed in northern Arkansas.