Complex sentence: A new volunteer formation called the Blue Legion ("Legión Azul") remained in combat on the Eastern Front, attached to 121st Infantry Division until March 1944, when it also was disbanded and the majority of the volunteers sent back to Spain.
Split into simpler sentences: A new volunteer formation called the Blue Legion ("Legión Azul") remained in combat on the Eastern Front, attached to the 121st Infantry Division until March 1944. It was then also disbanded and the majority of the volunteers sent back to Spain.