There is, I think, wide misunderstanding about the Christian doctrine of war. It might best be describes as being akin to the issue of homosexuality in Conservative Judaism, which has approved two conflicting rabbinical opinions on the subject for the faithful to choose from. (One continues the traditional position, while another removes all restrictions up to but not including rectal intercourse; a third responsum favouring complete abolition was rejected).
Two traditions can be found regarding war in the history of Christian thought, and so it may be said that a case may be made for both. Pacifism, however, originates with Jesus, and predates the Augustinian distillation of Just War theory. Moreover, the latter is limited in scope. Christians are not free to approve of just any war, if you will. In fact, most modern wars are total wars, which involve risk of loss of life to non-combatants. Right off the bat, they are disqualified from being salvaged by Just War. And so while it may be said that Christians are not bound to pacifism strictly speaking, the range of permissible belief is not so broad as commonly represented. To say that one who is not pacifist may be a Christian is not say that a Christian can support, for example, the ongoing war in Iraq, much less enlist for armed service. The days of the Major the Reverend (who retires, of course, to schoolteaching) are long gone.