If the goyish army will execute you for disobeying orders, it probably is acceptable to be medallel Shabbos (possible exception would be if they were targetting Jews in order to force Jews to go off the derekh – then one is required to die rather than do even the most minor averah).
In an actual situation of pikuach nefesh (actually quite rare, even for combat soldiers) you can preserve your own life, regardless (with exceptions that might involve avodah zarah or killing an innocent person, in which case preservation of your own life does not take precedence).
In a country like the United States or Israel, where a soldier disobeying order will at worst be thrown in jail, and probably given an undesirable discharge, there is never a heter to be mehallel Shabbos other than in a situation of pikuach nefesh.
Remember the typical sitation would be an order operate electric equipment or do paperwork in a training exercise or as part of routine garrison duty. Combat situations are rare even for combat soldiers.