Jephthah starts his negotiation with a crucial distinction: Israel didn't just grab land. What's the difference between taking land by force and stealing it?
When Jephthah says, 'Israel did not take away the land of Moab or the land of the Ammonites,' he's laying down a vital legal and historical argument. He's not saying Israel didn't conquer territory previously held by these nations. Instead, he's asserting that Israel only took land that was already conquered by the Amorites, specifically the land under King Sihon's rule.
Think of it like this: If a neighborhood has a disputed property line, and someone forcefully takes it, that's one thing. But if the previous owner of that property is dispossessed by a third party, and then a new group claims it based on that third party's conquest, it's a different legal standing. Israel is arguing they inherited land that was legitimately won from Sihon, who had himself taken it from others. This wasn't an act of aggression against Moab or Ammon directly, but a result of God's provision through conquest.