Complex sentence: Roderic was believed to have been killed, and a crushing defeat would have left the Visigoths largely leaderless and disorganized, partly because the ruling Visigoth population is estimated to have been a mere 1 to 2% of the total population.
Split into simpler sentences: Roderic was believed to have been killed, and a crushing defeat would have left the Visigoths largely leaderless and disorganized. This is in part because the ruling Visigoth population is estimated at a mere 1 to 2% of the total population.