The question is whether, if the work would not be done on shabbos, this might lead to serious danger to life. Basically the same criteria applying to anyone else commonly found working on shabbos, such as hospital staff, firefighters and the utilities (electrical, in particular – quite a lot of people depend on life-saving equipment at home that needs electricity).
Such people may be found in different branches than you commonly believe. I, for example, am an IT engineer working for an IT company that supplies absolutely critical equipment to, amongst others, sectors such as the government and medical. It’s public information (just checked) that amongst our customers is the Israeli Air Force. Imagine one of their critical systems would stop working on leil shabbos and they need someone to fix it *now* or, maybe, they won’t be able to operate the computers that keep the skies safe? (DISCLAIMER: I have not the slightest clue what the Air Force exactly does with the equipment the company provides – I’m just wildly guessing here!!!) Should we just not let the UAVs fly over Gaza that shabbos, thereby *not* being able to spot terrorists about to launch rockets on Sderot that might kill someone? Should we shut down the computers that analyze the radar images? Should we shut down the intelligence-gathering systems?
There’s a situation that is clear and obvious pikuach nefesh, isn’t it? So just to show, the military definition of ‘pikuach nefesh’ is not necessarily that of a combat soldier with a rifle on a battlefield. In fact, it could theoretically even involve *me* sitting right here in the UK working for an IT company. Pretty unlikely but possible. This also shows that one can hardly consider teshuvos from the 19th century as relevant to today’s situation.
Similarly to what I said, the same applies to cyber-warfare, for example. What if a giant attack against Israel’s (or the UK’s or the US’s) critical infrastructure takes place on shabbos and the mission-critical systems provided by some company don’t work? Should we then just allow the Iranian attackers to do whatever they want? It seems logical to me that the employees of that company must be available for assisting on shabbos if needed for the purpose of protecting the nation’s critical infrastructure.