Genesis 21:4 (JPS) And Abraham circumcised his son Isaac when he was eight days old, as God had commanded him.
Starting with Abraham, the Israelite men were circumcised and a boy at the eighth day after birth. It was commanded again in the desert through Moses.
Leviticus 12:3 (JPS) And in the eighth day the flesh of his foreskin shall be circumcised.
But the children born in the desert were not circumcised.
Joshua 5:5 (JPS) For all the people that came out were circumcised; but all the people that were born in the wilderness by the way as they came forth out of Egypt, had not been circumcised.
How is this possible? Did they disobey G-d and not circumcised their children in the desert?