The Midrash (Shir Hashirim Rabbah 2:7) tells us the reason for the oath against going up as a wall to take over Eretz Yisroel: “If so, why does the king moshiach have to come to gather the exiles of Israel?”
The Maharzu explains that it is moshiach’s job to bring all of Israel up together from the exile, and if, G-d forbid, they do this on their own, they will lose the redemption of the moshiach.
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