[...] what are the swords of strangers doing here? In order that the verdant plain be painted red with that barbaric blood? (20-22)
Petrarch is really good at showing us what Italy has lost because of the violence invited by warring nobility. In this case, he sets the natural beauty of the land against the foreign mercenaries who insist on bleeding all over it.
What fault, what judgment, or what destiny makes you harass your wretched neighbor, and broken, scattered fortunes persecute, and seek out foreign friends, glad to know that they shed blood and sell their souls for money? (57-64)
One major fault of the elite class is that they have no consideration for the lives they are destroying in their pursuit of war. They've forgotten the lower social classes are, in fact, people in their neighborhood—and people toward whom they have a responsibility. Petrarch also takes them to task for putting faith in people they've hired to murder their own countrymen.
[...] and this comes from the seed (even worse pain) of the unlawful people whose sides, as the book tells us, Marius so split open that memory of the deed has yet to fade, when he thirsty and tired drank as much blood as water from the river. (42-48)
Petrarch reminds his audience that Italy is now suffering as a kind of revenge for the butt-whipping that the ancient Roman Marius dished out to Germanic tribes. It might be ancient history, even back in Petrarch's day, but the outrageous slaughter can't have been forgotten by the current mercenaries.