By about 900 AD, the Holy Roman Emperors started to lose power over Northern Italy. The Counts of Canossa controlled most of Northern Italy: Florence, Pisa, Siena, Genoa. Around the year 1000 AD, a new city called Venice, in Northern Italy near Greece, became very rich by trading along the Silk Road. Venice was supposedly under the rule of the Byzantine Empire, but really Venice was strong enough that the Byzantine Emperors couldn’t tell Venice what to do.
After Matilda of Canossa died without children in 1115 AD, Northern Italy split up into a lot of independent city-states. This arrangement was like classical Greece, or West Asia in the Sumerian period. The cities were republics, with elected leaders. Like Venice, they traded with Islamic merchants, buying steel, sugar, paper, and cotton on the Silk Road, and selling wool cloth, tin, silver, and wine. The cities got rich from all the trade. The Italian cities were always fighting each other and with the Papal States to their south, as well as with the French to the north. The Holy Roman Emperor tried to get control of these North Italian cities, because they were so rich, but he never really managed it.
In the center, the Papal States were also pretty weak between 900 AD and about 1200 AD. Still they held onto the whole middle of the Italian peninsula. Mostly the Popes were able to stay in power because they had the help of the French kings. The French kings were afraid that the Germans (the Holy Roman Emperors) would take over Italy and become more powerful than France. So the French kings always helped the Popes when the Germans attacked them.
In southern Italy, the Normans, descendants of the Vikings, conquered the Islamic states by about 1100 AD and from then on, the Normans ruled Naples and southern Italy (and Sicily) for many years. Like the other ports further north, Naples was also an important and rich trading port. So in the High Middle Ages, too, Italy continued to be divided into three zones: North Italy with its independent cities, central Italy under the Popes, and southern Italy, under Norman rule.