Complex sentence: The historian Warren Treadgold, however, argues that Thomas stayed in the empire and may have even remained in active military service, and explains his obscurity by Thomas's association with Bardanes, which hampered his career.
Split into simpler sentences: The historian Warren Treadgold, however, argues that Thomas stayed in the empire and may have even remained in active service. Treadgold explains his obscurity by Thomas's association with Bardanes, which hampered his career.