Recall my critique of Hugh Elliot's idea that the Catholic Church may be likened to a ruler's army in its stress on mindless obedience.
I only want to add to it the C.S. Lewis' understanding:
Christianity does not think this is a war between independent powers.
It thinks it is a civil war, a rebellion, and that we are living in a part of the universe occupied by the rebel.
Enemy-occupied territory -- that is what this world is. Christianity is the story of how the rightful king has landed, you might say landed in disguise, and is calling us all to take part in a great campaign of sabotage.
As the song goes:
Onward, Christian soldiers!
Marching as to war,
With the cross of Jesus
Going on before. ...
At the sign of triumph
Satan's host doth flee;
On, then, Christian soldiers,
On to victory, etc.
Again, the metaphor extends this far and no farther. Christianity calls for self-sacrifice, not for crazy physical slaughter of innocents in your government's unjust wars unjustly conducted.