Some thoughts:

[A. On the non-pacifist side:]

(1) Killing to defend the innocent may be, if anything, _more_ justifiable
than killing in self-defense.  I can turn my _own_ other cheek, but I have
no right to turn someone else's.

(2) It seems to me that if Jesus had meant to teach pacifism, He would have
made His position more explicit.  He didn't tell the centurion to leave the
army, for instance; and the NT is full of military metaphors.

[B. On the pacifist side:]

(1) Apparently many early Christians refused to fight in the Roman army,
or stated that one should refuse if given a choice.  But it's not clear
whether they were objecting to war _per se_, or objecting to Roman policies.

(2) In modern warfare, it seems to be impossible to direct attacks only at
combatants.  Bombing, both conventional and nuclear, kills lots of civilians.

(3) It's hard to tell whether any _particular_ war is justified at the time.
Often it takes decades for the requisite information to become available
to the general public.

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