Rashi: They will die of thirst.

Radak: Without water, the soldiers will scatter to find water. Mo'av will come, and [Yisrael] will not be able to escape.

Malbim: He saw that he was foolish to take a crooked path in place of the straight path. Hashem put this in their hearts, so they will fall! Had he not called to Melech Yehudah for help, he would not have gone via Edom, for Edom was not under his control. Also this was from Hashem; he could have defeated Mo'av without asking for help!