‘ For it was of YHWH to harden their hearts, that they might come against Israel to battle, that he might destroy them utterly (devote them), that they might have no favour but that he might destroy them, as YHWH commanded Moses.'
This indeed was within YHWH's purpose. The offer of peace was made because it was humane, but the wickedness of their hearts was such that it was better that they were destroyed. And this was what YHWH had commanded Moses (Deuteronomy 7:2). The hardening of their hearts was an indication that in the end YHWH was seen as over all things, even men's thoughts. But He would not have hardened their hearts if they had not hardened their own hearts.