Flattered by futile error, little you see, thinking you see so much, for you seek love and trust in venal hearts— he with more followers is more surrounded by his enemies. (23-27)
Petrarch addresses the nobility responsible for inviting the mercenary soldiers into a tumultuous Italy. Basically he's saying, "What can you be thinking?! These guys are only out for the money!"
If by our very hands this has been done, then who will rescue us? (31-32)
It's a good question for the poet to ask. Since Italian nobility has asked foreign mercenaries to carry out the dirty work of in-house fighting, there's no easy way to ask them to leave. Also, what country or power will intervene to stop the madness, especially if they don't want to be targeted next? It's just not well thought out, at all.
Nature provided well for our condition when she raised up the screen of Alps between us and the German rage; but blind desire fighting its own good then managed to contrive a way to make this healthy body sick. (33-38)
So Nature was wise enough to protect Italy from foreign forces, but Italians themselves were not smart enough to take advantage of this by not inviting them in. Petrarch is calling out the nobility for their greed and anger, telling them that their own agendas are destroying the body politic.
Now inside the same cage the savage beasts there with the gentle flocks are nested, so the best are made to groan (39-41)
The poet uses an apt metaphor to describe the position of the everyday people ("the gentle flocks") who are trying to live with the violence and chaos brought on by warring bodies ("the savage beasts"). And yes, he's pretty much calling Italy a "cage" here.
It seems—who knows by what malignant stars— that now the heavens hate us, and thanks to you, to whom so much was trusted. (52-54)
Is it fate that has brought Italy to such a place? Or is it something more like the stupidity and shortsightedness of the nobility? Since Petrarch is pretty clear about who is at fault ("thanks to you"), we're going to vote for the second option.
[...] if that fury up there, that savage race, conquer our intellect, the sin is ours, and not the course of nature. (78-80)
Yes, this is Petrarch being xenophobic, but he kind of has good reason: he's only ever seen the Germans at their bloodiest. Notice that he's not shedding tears over them—he's disappointed in the Italian lords. No one else can be blamed for what the people suffer at the hands of warring noble houses.
Now you are here, but think of your departure: the soul, alone and naked, one day will come to face the perilous pass. (100-102)
Petrarch wouldn't have to remind the nobility to consider their deaths if they were being wise. This is, quite literally a last resort appeal to them to start behaving morally toward the people of the country, who are in their care.