Complex sentence: They belonged to the pro-papal elite of the Guelph faction and with an old military tradition not a few members of the family were considered the most valiant captains in numerous decisive battles of the Florentine Republic.
Split into simpler sentences: They belonged to the pro-papal elite of the Guelph faction. With an old military tradition, not a few members of the family were considered supremely valiant captains in numerous decisive battles of the Florentine Republic.