The difference now is that military service is entirely discretionary. I would contend that no real Christian has any business serving in the military to defend a country whose government is openly hostile to Christianity.
We are the salt of the world and were here to call sinners out of their darkness. Our charge IS to go into the world and try to be lights to it. The real problem isn't about Christians working within a pagan world. The real threat is unbelievers who infiltrate the church.
1 Cor:9 I wrote you in my letter not to associate with immoral people; 10 I did not at all mean with the immoral people of this world, or with the covetous and swindlers, or with idolaters, for then you would have to go out of the world. 11 But actually, I wrote to you not to associate with any so-called brother if he is an immoral person, or covetous, or an idolater, or a reviler, or a drunkard, or a swindler—not even to eat with such a one. 12 For what have I to do with judging outsiders?