Rashi citing Targum Yonasan: Say [to Ben Hadad].

Rashi: One who arms himself and descends to war should not boast like a Gibor who was victorious. The former does not know whether he will win!

Malbim: One who girds for war and considers all possibilities, i.e. perhaps he will be defeated, he fights with [proper] counsel and Gevurah. If while girding for war one thinks that he already won, usually he does not win. This is because (a) He did not properly prepare strategy and Gevurah for war. (b) Through this, his opponent casts his soul opposite him. I.e. you should have made Shalom with me. I accepted to be your slave, and it is a Safek if you will win!