What was their response?
Rashi (15,16): Indeed, we are cramped. Har Efrayim will not suffice for us, and we cannot conquer the Kena'ani (Eretz Perizi and Eretz Refa'im), for they are strong, and have iron chariots.
Malbim: (a) The Har will not suffice for us. (b) It is good that we leave it as a forest, to divide between us and the Kena'ani, who have iron chariots.