On November 5, 1862, Tappan received his commission as a brigadier-general and was transferred to the Trans-Mississippi Department under Major-General Sterling Price. He commanded his brigade at the Battle of Pleasant Hill, defending against Maj. Gen. Nathaniel Banks' Red River Campaign of 1864. His brigade was moved northward back into Arkansas to meet Major-General Frederick Steele at the Battle of Jenkins' Ferry. He also took part in Price's Missouri Expedition.