When all the Amorite kings across the Jordan to the west and all the Canaanite kings near the sea heard how the Lord had dried up the waters of the Jordan before the Israelites until they had crossed over, they lost heart and their courage failed because of the Israelites.
References for Joshua 5:1
At that time the Lord said to Joshua, "Make flint knives and circumcise the Israelite men again."
So Joshua made flint knives and circumcised the Israelite men at Gibeath-haaraloth.
References for Joshua 5:3
This is the reason Joshua circumcised [them]: All the people who came out of Egypt who were males-all the men of war-had died in the wilderness along the way after they had come out of Egypt.
Though all the people who came out were circumcised, none of the people born in the wilderness along the way were circumcised after they had come out of Egypt.
For the Israelites wandered in the wilderness 40 years until all the nation's men of war who came out of Egypt had died off because they did not obey the Lord. So the Lord vowed never to let them see the land He had sworn to their fathers to give us, a land flowing with milk and honey.
References for Joshua 5:6