Art thou anything better than Balak? — Literally, Are you the good, good in comparison with? It is one of the Hebrew ways of expressing the superlative. Jephthah here argues from prescriptive right, which even the contemporary king Balak had not ventured to challenge, showing, therefore, that he admitted the claim of Israel, deadly as was his hatred against them.
Did he ever fight against them? — This may seem at first sight to contradict Joshua 24:9. There “Balak the son of Zippor arose and warred against Israel”; and we might infer that it was in some Moabite battle that Baalam had been slain (Numbers 31:8; Joshua 13:22). But this would not affect Jephthah’s argument. Balak had fought against Israel out of pure hatred, not from any pretensions to claim their conquests from them.