Judges 12:2

And Jephthah said unto them, I and my people were at great strife with the children of Ammon; and when I called you, ye delivered me not out of their hands.

Cross-reference

Judges 11:12-33 recounts Jephthah's failed diplomacy with Ammon — the same history he summarizes here to defend himself.

Numbers 21:24 shows Israel taking the land from Sihon — the very victory Jephthah references to prove he didn't fight Ephraim.