What stirred them to fight now?
Malbim (1): (a) They heard that Yehoshua Lachad (captured through strategy) Ai. These kings' strength is via cunning and strategy. (b) As they did to Yericho (totally destroyed it), so they did to Ai. Those who conquer lands normally destroy the first city, to impose fear. Yisrael did so also to the second; this shows that they intend to do so to all of the cities. (c) Giv'on made Shalom with Yisrael, due to fear. This put fear in the hearts of all [Kena'anim]. (d) Giv'on are now among them. Now many will make Shalom with Yisrael and pay taxes. Also, now Yisrael have settled cities to supply them with food, shelter and refuge.