Joshua 5:4

And this [is] the cause why Joshua did circumcise
Or the reason of the command given him to circumcise the children of Israel at this time, namely, what follows:

all the people that came out of Egypt [that were] males, [even] all
the men of war;
meaning such that were twenty years old, and upwards:

died in the wilderness, by the way, after they came out of Egypt;
not directly, but in a course of forty years, as they journeyed through the wilderness; this is to be understood with an exception of Joshua, Caleb, Eleazar but then there was a large number who were under twenty years of age, that came out of Egypt, and were now living.