Responsa su Salmi 110:8

All the cases mentioned deal with one's personal obligations and, except directly by way of analogy, do not concern the personal danger in fulfillment of one's duty to the government. There is, for example, no direct law which makes use of the verse in Psalm 110,3, Amcha nedavos b'yom chaylecho, which Rashi and Kimchi understand to mean that the people gladly volunteered for war service in the wars of King David. Of course in any obligatory war no one was freed from the danger of war but all were obliged to go. (M.Sota VIII, 7, Maimonides, Hil. Melachim V,1-2.) All this refers to the wars of Israel. Of course there is, as has been stated, no question as to the duty to obey the command to serve in the armies of the lands of our citizenship, but to what extent is it a moral or religious duty to take on voluntarily the dangers of such war?