Complex sentence: Milites were the main footsoldiers of the praetorian guard, as part of the praetorian guard they were selected from the best roman legionary troops and were, although only slightly above them, elite.
Split into simpler sentences: Milites were the main footsoldiers of the praetorian guard. As part of the praetorian guard they were selected from the best roman legionary troops and were, although only slightly above them, elite.