Or HaChaim on Deuteronomy 29:21/אור החיים על דברים כ״ט:כ״א

1 ואמר הדור האחרון..וראו את מכת הארץ, "The later generation will say,…when they see the destruction and the conflagration, etc." Clearly, the Torah envisages that such punishment will not be something isolated. Why does the Torah mention the reaction by a later generation viewing the destruction of the Holy Land before mentioning the fact that they have seen it? The Torah should have written: "the later generation will see what destruction happened to this country and they will ask what caused it." Besides, the word ואמר is totally superfluous here seeing the Torah repeats in verse 23 that "they will say" i.e. all the other nations who came face to face with the destruction in the Holy Land. Furthermore, why did the Torah begin the verse in the singular, i.e. ואמר, only to switch to the plural, i.e. ואמרו כל הגוים, "and all the nations will say?"

2I believe the best approach to understanding the sequence in our verse is as follows: "the later generation will say, etc." who is this later generation? Your children. The Torah had to spell this out seeing that other nations also have later generations. The Torah adds that not only your own children will raise this question, but even the Gentile who lives nearby and the nations who come from afar. וראו When all these nations from near and far will observe how this land has been devastated, they (too) will say: "what has caused this destruction?" It is taken for granted that your children who live in the land did not have to come and see the destruction. This is why the Torah could commence with describing only their reaction. The reason the Torah speaks of כל הגוים, "all the nations," is that those who have seen the desolation in the Holy Land will tell all those who have not seen it with their own eyes.

3The desolation described in this verse refers to the people of whom the Torah spoke in verse 17, the ones who decided to become idolators. The curses which had been mentioned immediately prior to this verse referred to the individual who tried to outsmart the Torah by hedging his acceptance of the oath of the covenant prior to affirming it with his mouth. The warning that G'd would erase that man's name applied only to the individual described in verse 18. Exile and desolation of the land of Israel are the result of idolatry.