“ Come up to me, and help me, and let us smite Gibeon, for it has made peace with Joshua, and with the children of Israel.”
The first plan was to smite Gibeon. In their view what Gibeon had done had been an act of treachery against them, and their aim was to weaken the new alliance (as they saw it. They would not know the full story) and would be a warning to other cities not to ally themselves with Israel.