He’s just a little worried; just a little, mind you. That voice that’s telling him that it was a bad idea to go to war against this nation doesn’t exist, because he can’t hear it over the sound of his own words. He can’t hear it over the silence of the dry river and the panting of his army.
Instead, he tries to break open the minds of his enemy with his words, casting stones and spells because he knows that all these things hurt. He knows, because all he hears is silence. Not a word of protest or defiance or denial reaches his ears, and he thinks he’s winning.
So he allows his pride to take over and his fear to raise him up, and tells their God what He can and cannot do.