1)
What was "Al Pi Hashem"?
1.
Rashi: All the evil that came to Yehudah via these troops.
2)
Was it purely due to Menasheh's sins?
1.
Radak: If not for Menasheh's sins, Hashem would have pardoned the sins of those after him. Due to his sins, it was decreed to destroy Yerushalayim.
