O, my own Italy, though words be useless to heal the mortal wounds I see covering all your lovely body, I wish at least for my sighs to be one with Tiber's hopes and Arno's as well as Po's, where I sit sad and grieving.
Petrarch opens the poem with an apostrophe (no, not the punctuation mark), addressing an Italy suffering from the "mortal wounds." Hmm, it sounds like bad times for his home country.
In fact, he personifies Italy in lines 2-3, giving it a body that can be wounded and bleed.
We can see that Petrarch is uber-attached to his country, because he figures her as his beloved—he sighs for her suffering and wants to unify his desires with that of his land.
Petrarch uses the names of the three major rivers of Italy (Tiber, Arno, Po) to represent different regions of the country. In this way, he shows that he's interested in a wider Italian identity rather than a cliquish attachment to cities or communes.
He tells us that he sits near the Po, a river in the north of Italy, as he composes this poem. We don't yet know the exact nature of his sorrow, but he's pretty bummed.
Ruler of Heaven, I beg now that mercy which once brought you down to earth turn you again to your dear, holy land. You see, my gracious Lord, what trivial reasons cause so cruel a war.
Petrarch opens line 7 with an invocation to God ("Ruler of Heaven"). Invocations are common in classical literature (all epics, like The Odyssey, had them) and it looks like Petrarch has an interest in bringing up the classical past.
He's basically asking God to show mercy to Italians here—but not just garden-variety mercy.
Petrarch needs the heavy-duty pity that he believed God showed by when he sent Jesus to earth.
In these lines, we get a better sense of what's up: a senseless and bloody war.
Those hearts enclosed and hardened by fierce and haughty Mars, open them Father, free and soften them, and there the truth divine be heard through me, unworthy as I am.
Petrarch uses metonymy in line twelve (where "hearts" stand for the will of the warriors) to help him talk about the stubbornness of those inflicting hurt on the body of Italy.
And who is responsible for this hardening of hearts? Why, it's Mars, of course. To be fair, though, he's the Roman god of war—it's not like he can help it.
Petrarch continues with his supplication, asking God to do something to change the hostility factor.
In fact, he has one idea how to do it: he could write a poem that speaks the truth to these mules and see if that works.
Petrarch falls back on an old trick in line 16—false modesty—to make it seem like he's really doing this as a service to Italy and God. He's unworthy, but it's his duty as a poet to speak the truth.