Ramses III was not the only ancient conqueror who castrated his conquered enemies (see Niehaus, Ancient Near Eastern Themes in Biblical Theology, 143). He destroyed the household of the conquered enemy by making it impossible for the household to reproduce. They were made eunuchs for the kingdom of Ramses.

Circumcision is a symbolic castration. It removes the foreskin as a sign that it is removing the whole. By circumcision, Abraham’s children were marked as the conquered, vassals to Yahweh. 

By circumcision too, their households were symbolically cut off. Abraham became Yahweh’s eunuch, without hope of progeny, without future. The only possible future was the one that would come through the power of Yahweh’s Spirit, as a gift beyond flesh.