Complex sentence: The Guelphs and Ghibellines were factions supporting, respectively, the Pope and the Holy Roman Emperor in central and northern Italy during the 12th and 13th centuries, these two parties were particularly important among the internal policy of the Italian city-states.
Split into simpler sentences: The Guelphs and Ghibellines were factions supporting, respectively, the Pope and the Holy Roman Emperor in central and northern Italy. During the 12th and 13th centuries, the split between these two parties was a particularly important aspect of the internal policy of the Italian city-states.