Health,
1. The tiny yishuv could not possibly have withstood the Arabs in 5708 and 5727 or take in double it’s size in ten years without Hashem’s help.We will accept whomever He sends as shelichim (Me’ila 17a).
2. A melech is not needed for a milchemet mitzva. This is proven by the fact that various judges conducted wars even before they became judges. Anyway, according to Rav Kook any leader accepted by the people has the din of melech for these matters. This is also the mashmaut of the Netziv on the mitzva of appointing a king.In any case, all go into the army in a milchemet mitzva (Rambam Hilchot Melachim 7:4). The leader (whatever his technical title is) compels them if necessary (ibid 4:2).