The Ethiopians (Zephaniah 2:12).
“You Ethiopians also, you will be slain by my sword.”
This brief reference takes us by surprise. The Ethiopians (Cushi) would probably be the furthest nation south known to Zephaniah. But we would have expected more comment. This suggests that rather than having any particular reason for targeting the Ethiopians he was just deliberately thinking of as far south as possible. Basically he was indicating that almost the whole world of his day was to be included in the coming catastrophe. The ‘Ethiopians' were more in the area of the Sudan on the upper cataracts of the Nile, although they may have extended into Ethiopia.
It has been suggested that Egypt was in mind for they had been ruled by Ethiopian kings for sixty years in the not too distant past (up to 664 BC), with the reference being contemptuous, ‘you Ethiopians'. But it seems unlikely that if it were Egypt the reference would be so brief, whereas Zephaniah probably knew little about the Ethiopians apart from the fact of their existence.
Nebuchadnezzar would in fact conquer Egypt and include Ethiopia in his conquests.