To harden their hearts (cf. Exodus 4:21; Exodus 7:23). Muller, 'Christian Doctrine of Sin,' 2.412, says that "Scripture never speaks of God's hardening men's hearts, save in connection with His revelations through Moses or Christ."
This passage evidently had not occurred to him when writing. His explanation of the difficulty is hardly satisfactory. We are not to suppose that the free will of the Canaanites was in any way interfered with.
God no doubt left them to themselves as the due punishment of their iniquities. Sin in general, by God's own appointment, and especially the sensual sins in which the Canaanites were steeped, has a tendency to produce insensibility to moral or even prudential considerations, and to beget a recklessness which urges on the sinner to his ruin.
Some have argued that had they all come, like the Gibeonites, as suppliants, they must all have been massacred in cold blood. But this is not likely. Rather we must imagine that God foresaw that they would not believe the signs He would give in favour of the Israelites, and that by meeting them in battle they brought a swift and speedy destruction on themselves.