At that time the Lord told Joshua, “Make flint knives and circumcise the Israelites once again.” 1
This is why Joshua had to circumcise them: All the men old enough to fight when they left Egypt died on the journey through the desert after they left Egypt. 1
He replaced them with their sons, 1 whom Joshua circumcised. They were uncircumcised; their fathers had not circumcised them along the way.