Assyrian punishment

As a reminder that things would be different under their new regime, all the males of a captured nation were usually circumcised in a public ceremony. The foreskins were collected in baskets and taken to court scribes for counting. These numbers were then used to calculate tribute and taxes. (It was a rather simple accounting procedure when you think of it and a very effective object lesson!) Collecting foreskins wasn't that unusual. Sampson collected a basket full from the Philistines he dispatched to prove he had won the battle.

With Hebrew captives, or with previously conquered nations, circumcision was a redundancy they could hardly afford -- it already having been done! That's probably why their humiliation was to only have their private parts manhandled by the Assyrian barbers out in front of God and everybody!