John Calvin Commentary Joshua 5:1

"And it came to pass, when all the kings of the Amorites, that were beyond the Jordan westward, and all the kings of the Canaanites, that were by the sea, heard how that Jehovah had dried up the waters of the Jordan from before the children of Israel, until we were passed over, that their heart melted, neither was there spirit in them any more, because of the children of Israel." — Joshua 5:1 (ASV)

And it came to pass when, etc. The recognition of God's fearful power affected them so much that they were astonished and fainted with terror, yet it did not incline their minds to seek a remedy for the evil. Their hearts melted; for, lacking counsel and strength, they did not rouse themselves to action, but in their defiance, they remained as hard-hearted as before.

We have already seen elsewhere how unbelievers, when stricken with fear, do not stop wrestling with God, and even when they fall, continue fiercely to assail heaven. Therefore, the dread that should have urged them to caution only served to make them rush headlong. They were, however, terrified from above for the sake of the people, so that victory might be more easily obtained and the Israelites might be emboldened when they saw they were dealing with an enemy already broken and stricken with dismay.

Thus God made allowance for their weakness, as if He had opened the way by removing obstacles, because they had otherwise shown themselves to be more sluggish and cowardly than was fitting. In essence, then, before the conflict began, the enemy was already routed by the terror that the report of the miracle had inspired.