After talking about our glorious redemption through a clear Divine battering of our enemies, the passuk (39:43) forecasts that ‘The other nations will realize that it was Bnei Yisrael’s sins that caused them to be exiled in the first place.’ Why should our redemption prove that galus was a result of our sins?
The Metzudas David explains that once the nations see Hashem’s miracles, they will realize that our prolonged exile was not due to Hashem’s ‘lack of power’ to redeem us, rather, it must have been because of our sins.
Perhaps we can offer another explanation of this passuk, based on the Maharal.
The Maharal writes (Netzach Yisrael perek 1) that exile and redemption prove each other, for it is not normal/natural to have a nation so persecuted and downtrodden in such an extensive way. Given that unnatural occurrences by definition cannot last (if they did, they would be ‘natural’ and normal), exile proves that Bnei Yisrael will one day return to their essence as free people: redemption. Accordingly, once the other nations witness our redemption, they will look back at our exile and wonder why we were in exile for so long. If, indeed, the norm of the Bnei Yisrael is to be redeemed, why were we in exile for so long? They will therefore conclude that it must have been due to our sins.
May we merit the arrival of Moshiach and the third Beis Hamikdash speedily in our days.