Jephthah presents the king of Ammon with a sharp historical and theological argument, contrasting the powerlessness of foreign idols with the strength and providence of God. In his message, Jephthah brings up Chemosh, the idol worshipped by Moab, addressing the Ammonite king with a tone of clear mockery [מצודת דוד, רלב״ג].
He reminds the king of past events to prove his point. Previously, the Amorites conquered this territory directly from Moab. During that invasion, Chemosh was entirely unable to save his people or defend their land [רלב״ג, רד״ק].
Building on this history, Jephthah draws a distinction between the violent uprooting of a nation and the rightful taking of an inheritance [מצודת ציון, רד״ק]. He sets up a simple equation: the Ammonites serve Chemosh and must therefore settle for whatever their idol is capable of providing, even if he failed to protect their borders in the past. In stark contrast, the Israelites rely on the wonders and direct providence of God [מלבי״ם]. Because God actively drove out the Amorites to give the land to the Israelites, whatever God has granted them remains their just and rightful inheritance [רד״ק, ביאור שטיינזלץ].