Wa `alaykum As-Salamu wa Rahmatullahi wa Barakatuh.

The opinion, which is more akin to the sound juristic view, is that a Muslim shouldn’t indulge in a war against his fellow Muslim brothers, and he may justify his position by asking for a leave or (a temporary) exemption from the military as the true conscience of a Muslim dictates that he shouldn’t indulge in killing a fellow Muslim brother without a justifiable reason.

However, if there is no way but to participate, then a Muslim can join the rear to help in military service (i.e. not to participate in face-to-face confrontation).

This is the case of someone who is already enrolled in a non-Muslim army.

For one who is seeking to work in a non-Muslim army, I think he should not do that since it may cause him to fight or confront a Muslim or cause a great harm to Muslims.

Almighty Allah knows best.