‘ And all the cities of the tableland and all the kingdom of Sihon, king of the Amorites, who reigned in Heshbon, whom Moses smote with the chiefs of Midian, Evi and Rekem, and Zur and Hur, and Reba, the princes of Sihon who dwell in the land. Balaam also, the son of Beor, the soothsayer, did the children of Israel slay with the sword among the rest of their slain.'
For ‘cities of the tableland' compare Deuteronomy 3:10. This was clearly a local name for the whole area. (Contrast ‘the cities of the plain' - Genesis 19:29).
The description is a brief summary of larger events occurring at different times, the slaying of Sihon and his forces at one time (Numbers 21:21-31) and the slaying of the ‘leaders of Sihon' and Balaam at another (Numbers 31 where they are also named). It would appear that these Midianite chiefs were vassals to Sihon, probably due to receiving certain rights to the use of lands for grazing, who were not present when Sihon was slain. Their subsequent appearance to avenge Sihon was met with by the denunciation of YHWH and their defeat at the hands of Israel (Numbers 31). The word translated ‘princes' is a rare one occurring only five times in the Old Testament. See especially Psalms 83:11 where it refers to Midianite leaders as here. Thus it was seemingly a term especially applied to Midianite leaders. The other references are Ezekiel 32:30 where it refers to vague ‘princes of the north'; Micah 5:4 where it is subordinate leaders to the shepherds; and Daniel 11:8 where it has a different meaning altogether.
Balaam was said in Numbers 24:25 to have ‘returned to his place' (see Numbers 22:5). However it appears that either he dwelt among the Midianites or was recalled by the Midianites to obtain revenge for Sihon for here he was slain along with them. The slaying of such a powerful soothsayer (compare Deuteronomy 8:14), who was made wealthy by divining on behalf of people against their enemies, was looked on as a great feat.