Nature provided well for our condition when she raised up the screen of the 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.
Stanza 3 opens with a personification of Nature (Mother Nature, if you will), who apparently has better maternal instincts toward Italy than its nobles do. She, at least, has put the Alps in a convenient location to protect Italy from foreign invasion.
You might note that the xenophobia we saw back in stanza 2 has returned: Petrarch does not like or trust the Germans. It looks like we're about to find out why.
It seems that the Italian elite has managed to take all the great natural advantages of the land and turn them on their heads. They've got to cross those protective Alps to bring violence and warfare to the land.
That "lovely body" of Petrarch's beloved Italy from stanza 1 makes an appearance again here.
He wants to emphasize that the fallout of warfare touches every aspect of their lives—the people and the land as well.
Now inside the same cage the savage beasts there with the gentle flocks are nested, so the best are made to groan;
In this startling metaphor, Petrarch reverses the idyllic prophecy from Isaiah (you know, "the calf and lion and sheep in one dwelling place" thing?). Nothing good can come from putting savage beasts in with gentle flocks, yo.
Just in case you were wondering, the "savage beasts" are the mercenary troops—they might also be the Italian lords who don't seem to care about the Italian people. The Italian people get the unhappy role of "gentle flocks."
Petrarch is pretty taken up with the idea that "Latin blood" is the best, so when he talks about the best being made to groan here, he's talking about the Italian people. FYI: "Latin" here refers to the people that spoke Latin (i.e., the people of Rome, both empire and republic).
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 the memory of the deed has yet to fade, when he thirsty and tired drank as much blood as water from the river.
Now we're getting into it. Petrarch tells us here why the whole situation is just so, so bad: the current mercenaries are the descendants of the mortal enemies of Rome.
Petrarch's quick reference to Marius brings up Rome's bloody involvement in the Cimbrian or Cimbric War, when the Roman consul slaughtered the aggressive Germanic tribes. (Take a look at our "Shout Out" section for more on this battle.)
Petrarch's point in bringing this up is to show how cosmically stupid the Italian nobles are for paying the Germans to bring weapons into Italy. That's grudge fodder right there.