What did the kings hear?
Malbim #1: They heard that Yisrael did not offer Shalom to Yericho and Ai (rather, they simply fought and defeated them); they thought that Yisrael will not accept nations that come to make Shalom with them.
Malbim #2: They heard that Yisrael needed war strategy. This invigorated them [to think] that it will be a natural war 1 , therefore they gathered together to fight.
The verses (8:22-25) imply that Yisrael killed a city of 12,000 without losing a man! Even though Yisrael used a clever strategy, and Anshei Ai were dispirited upon seeing their city afire, it is a miracle that they could not kill one Yisrael! Perhaps the kings did not hear this. Or, they heard that Yisraelim died in the first battle, and did not realize that it was due to sin. (PF)