Complex sentence: Abu Musa led the conquest of Khuzestan from 639 to 642 and was ordered by Umar to aid Uthman ibn Abu al-'As, then fighting Iran from a new, more easterly "misr" at Tawwaj.
Split into simpler sentences: Abu Musa led the conquest of Khuzestan from 639 to 642. After this, 'Umar ordered him to aid'Uthman ibn Abu al-'As, then fighting Iran from a new, more easterly "misr" at Tawwaj.