1)
What is the meaning of "bi'Fro'a Pera'os b'Yisrael"?
1.
Rashi: When [the enemy] made breaches] in Yisrael [for they abandoned Hashem].
2.
Radak: When vengeance was taken in Yisrael.
3.
Malbim: At a time when Yisrael is breached, either because they breached Musar, or due to weakness...
2)
What is the meaning of "b'Hisnadev Am Borchu Hashem"?
1.
Rashi: When the people volunteered to repent, they blessed Hashem for the salvations.
2.
Malbim: If people are willing to be valiant [at such a time], this is not natural, rather, due to Hashem. Therefore, they blessed Hashem.
