Facing a threat of war, Jephthah delivers a calculated historical and legal defense to the Ammonite king. Instead of rushing into battle, he completely rejects the enemy's claim to the land, firmly establishing Israel's moral and legal rights. The primary approach among commentators is that his main goal is to prove the Israelites never took any territory directly from Ammon or Moab. When the Israelites arrived in the region, the land in question was no longer under Ammonite or Moabite control. Instead, it was ruled by Sihon, king of the Amorites, who had captured it previously. Therefore, Israel won the land from the Amorites, taking absolutely nothing from Ammon or Moab [מצודת דוד, אברבנאל, מלבי״ם]. Furthermore, the Israelites had actively gone out of their way to bypass the borders of Edom and Moab to ensure there would be no conflict whatsoever [מלבי״ם, אברבנאל].
This historical reality raises a question about the Ammonite king's underlying logic for demanding the territory back. His claim rested on the legal theory that standard military conquest by nations does not grant permanent ownership of land. In his view, Sihon was merely a thief, meaning the land legally still belonged to its original owners, Ammon and Moab. Jephthah counters this by explaining that this particular conquest was an exception, orchestrated directly by God. God specifically decreed that Sihon would capture the land from Ammon and Moab in order to transfer it into Amorite hands. This divine maneuver allowed the Israelites to eventually inherit the territory legally, without violating God's explicit command that forbade them from taking land directly from Ammon and Moab [צוארי שלל, חומת אנך, אלשיך].
Although the Ammonite king was only demanding the return of his own nation's land, Jephthah specifically includes Moab in his defense. This is because the exact region under dispute, stretching from the Arnon to the Jabbok rivers, originally belonged to Moab before it fell to Sihon, and the area was still known by their name [רד״ק, אברבנאל].