One of the constants of Medieval Europe was the tension between the papacy and the Holy Roman Empire, which was not holy, Roman, or an empire. Sometimes this tension manifested itself in the form of excommunications going both ways, and other times it manifested as Italian cities who supported one side or the other going to war over ridiculous things.
I will now summarize this historical event after the manner of the Internet.
Bologna: "I has a bucket."
Modena: "I'm in ur base, takin' ur buckets."
Bologna: "No! They be stealin' mah bucket!"
*Warfare ensues. Thousands die.*
Modena: "All your bucket are belong to us."