Judges 11:13
And the king of the children of Ammon answered unto the messengers of Jephthah, Because Israel took away my land, when they came up out of Egypt, from Arnon even unto Jabbok, and unto Jordan: now therefore restore those lands again peaceably.
The demand to give back the land from Arnon to Jabbok and to Jordan belongs to the messages exchanged before Jephthah's war against the Ammonites. That war came after eighteen years in which Israel had suffered under Ammonite oppression. Its result was the ending of Ammonite power over Israel.