This question is not a historical one but a contemporary one as well, since in Israel jews serve in their own army and do have to fight during Saturdays. In general life and death situations precede the sanctity of sabbath, meaning if you have to save a life, must do whatever it takes to do that, even if it means to light a fire, do manual work and so on.

So in military service and in war in general, operational needs override the commandments that deal with sabbath. One can go on patrols, one can talk on the radio during patrols (if its essential, idle chatter is not included), and of course when it comes to combat itself. "Voluntary war" (a war that is initiated) is also permitted in sabbath even if it does not answer an immediate threat.

That being said, there are some limitations, a siege for instance has to start at least three days before sabbath.

EDIT: added detail