If you understand what Jesus meant when He said "NO more of this" (one of the last chapters of Luke) to His disciples when Peter attacked one of the guards that came for Him; and if you understand Christ's teaching of turning the other cheek after attack; or if you understand the voices (and advert placements) of the numerous dying innocent women and children especially begging for your mercy while you wage war at them; and if you understand the word "If you kill by the sword you'll die by the sword(note that this isn't a physical sword or human sword or this lifetime sword that Jesus is talking about, for there surely would be unrepentant murderers who have died peaceful deaths in human history. So since the Lord isn't a liar, this can't be a during-this-life "sword" that He spoke of)" said by our Lord; and most shamefully to us Christians, if you understand the premise behind Muhammad Ali (a convert to Islam from Christianity) rejecting to fight in the Viewtnam war (an act that landed him in prison); I guess you should agree with me that the military, or attack or extermination of human life for whatever reason is unchristian.
Remember that the presidents or people sending us to wars mostly always do it for selfish and malignant purposes(watch Fahrenheit 9/11; a documentary criticizing Bush's wars in the Middle East).
The apostles from their acts have shown us that there are points where we may reject the orders of our earthly leaders, namely: when their orders contradict God's will. (Check the story of Peter and John before the leaders of Israel; early part of the Acts of the Apostles)
May God's grace be with all who love our Lord Jesus. In Jesus name. Amen