What was the question "b'Mi"?
Malbim: Who will be at the front in the war?
Who are "Na'arei Sarei ha'Medinos"?
Rashi: They are children of mixtures. All officers of other nations put their sons under [Achav's] hand as guarantors that they will not rebel.
Radak: They are the children of the great officers. The custom is that they grow up in the king's Chatzeros. The Navi taught that you need not bring [foot] soldiers and cavalry from outside; Na'arei Sarei ha'Medinos will suffice.
Malbim citing Mahari: They are the servants of the officers, who are lowly and few.
What is the meaning of "Mi Yesor ha'Milchamah"?
Rashi: Who will arrange to command it?
Radak: Who will arrange the chariot? The head of the war is called Oser, for he begins, and the nation after him.
Malbim: Who will count the soldiers and conduct them?
Why did Achav ask who will command the war? Surely it is he - he is the king!
Radak: Because he was an idolater, and Hashem was doing a miracle for Yisrael, he thought that Hashem will not do it if he is there.