What does Judges 11:28 mean?
The king of Ammon has been thoroughly refuted. He claimed to be attacking Gilead to regain territory lost to Israel during the time of Moses (Judges 11:13). Jephthah's responses came in several categories. Historically, the Ammonites never possessed the land at all (Judges 11:18–19, 22–23). Theologically, God gave the land to Israel when they were attacked by the resident Amorites (Judges 11:15–17, 20–21, 24). Personally, greater kings than Ammon's current ruler knew not to go to war for this land (Judges 11:25). Chronologically, it's been centuries since Israel arrived and no attempt has been made to retake the region (Judges 11:26).The enemy ruler does not listen. It's entirely likely his claim was openly fabricated. He has already mounted his war effort (Judges 10:17–18; 11:4). Jephthah is the one who sent messages asking for a reason for the coming attack (Judges 11:12–13). The Ammonite king responded, but he doesn't feel the need for a good excuse. He believes he will simply defeat the people of Gilead and take their land, whether his cause is just or not.
From Ammon's perspective, there were reasons to confidently ignore Jephthah's appeal to the Lord's authority. The Ammonites and Philistines have been oppressing Israel for eighteen years at this point (Judges 10:6–8). They have no reason to assume Israel suddenly became strong enough to resist. Also, until very recently, the Israelites had been worshipping the same false pagan gods as everyone else. It's understandable that the king of Ammon would care little about a threat on behalf of a God whom the Israelites so willingly ignored.
For their part, Jephthah and the Israelites are hopeful the Lord would respond to their repentance, delivering them from their enemies once more. Confirmation of this will not come until after Jephthah has taken power and he is inspired to launch a counterattack (Judges 11:29).