Why did they want to fight Giv'on?
Malbim: Since all the kings gathered together and agreed to fight Yisrael, Giv'on breached the 'law' of the land and harmed the Klal via helping the enemy.
Radak (9:2) explained that they gathered due to (and hence after) the episode with the Giv'onim. He must explain that it was to dissuade others from making Shalom with Yisrael. (PF)