The theology of warfare

I regularly come across either serving or veteran military personnel in these atheist v theist groups who also identify as Christian. As a rule such people express pride in their military credentials and some even seem to assume that they deserve special consideration or respect as a result. This has always confused me.

How can a Christian join an organisation in which they will be expected to kill (or support the killing of) other people simply because a politician decides it’s a good idea? As we all know, military interventions are not always motivated by noble intentions.

I don’t suggest that military service isn’t courageous, even heroic and I fully understand that it genuinely can be necessary (although it often is not). It’s the decision to ‘join up’ that I find confusing.

How is a commitment to take the lives of unknown others at some future date for as yet unknown reasons, ordered by currently unknown politicians with equally unknown motivations in any way consistent with Christian values?

How do Christians justify their commitment to kill people because some politician (whose motives may be very reprehensible) tells them to?