What does Judges 11:20 mean?
Through messengers, Jephthah is describing everything Israel did to try to avoid conflict while crossing over the Jordan River during time of Moses. He is delivering this history lesson in response to the Ammonite king's wrong assertion: that Israel stole the land of Gilead from Ammon (Judges 11:12–14). Jephthah's main point has been that the Ammonites were not even occupying the land that close to the Jordan (Judges 11:15–19).Instead, the land was then occupied by the Amorites, an entirely different people. The Israelites wanted to pass through the land to get to the other side of the Jordan, so they sent a message to Sihon, king of the Amorites, asking permission. Sihon never answered the request. He did not trust the Israelites, assuming they were there to take the land from him. Instead of answering their question, he quickly gathered his army at Jahaz, and attacked the Israelites (Numbers 21:21–23).
Jephthah is proving that the king of Ammon was not only wrong about Ammonites occupying the land at the time, but he was also wrong about who attacked whom. It was the local Amorites that attacked Israel, not the other way around.