All the Amorite kings on the west side of the Jordan and all the Canaanite kings near the sea heard that the LORD had dried up the water of the Jordan before the Israelites until they had crossed over. Then their hearts melted. They lost all courage because of the Israelites.
References for Joshua 5:1
At that time the LORD said to Joshua, "Make yourself flint knives. Circumcise the Israelites for a second time."
References for Joshua 5:2
So Joshua made flint knives for himself. He circumcised the Israelites at Foreskins Hill.
This is the reason Joshua did so: All the people who went out of Egypt, that is, all the men who were soldiers, had died in the desert on the way after they left Egypt.
All the people who went out were circumcised. But none of the people born in the desert on the way after they had left Egypt had been circumcised.
This was because the Israelites journeyed forty years in the desert until the whole nation died off. These were the men old enough to fight who went out from Egypt and who hadn't obeyed the LORD. The LORD had pledged to them never to show them the land that the LORD had pledged to their ancestors to give us. It is a land full of milk and honey.