What does Judges 11:13 mean?

The king of the Ammonites answered Jephthah's messengers, 'When Israel came out of Egypt, they took my land from the Arnon River to the Jabbok and the Jordan. Give it back peacefully, and I'll stop.' Judges 11:13 - Modern Text Bible

(The Ammonite king claims Israel stole his land centuries ago, and says he'll stop fighting if they return it.)