"Now those who wage war are threatened by Our Lord with punishment, according to Mt. 26:52: "All that take the sword shall perish with the sword." Therefore all wars are unlawful."
Let's expand the biblical passage a bit, Mt 26:51-54
" With that, one of Jesus' companions reached for his sword, drew it out and struck the servant of the high priest, cutting off his ear. Put your sword back in its place, Jesus said to him, for all who draw the sword will die by the sword. Do you think I cannot call on my Father, and he will at once put at my disposal more than twelve legions of angels? But how then would the Scriptures be fulfilled that say it must happen in this way?"
Jesus objected to the use of force here, because it was not His will to resist the Sanhedrin guards. His comment that those that live by the sword will die by it can be understood as follows.
God became man to come here and teach, knowing full well He would be killed by those that are fundamentally opposed to His teachings. Those opposers are those that live by the sword. They are the ones that coerce and usurp the wills of others, just as satin had done.
Living by the sword denotes ones will is coerced on others. It is in direct disobedience to His command that one love ones neighbor as one's self. The free will that He gave as a gift to all is not to be usurped by those more proficient in the art of coercion. Death is damnation itself and in order for that to occur the damned must reject the Holy Spirit. That fact of rejection is known only to God, it is His determination and judgement.
War waged in self defence, the defence of Freedom and the defence of other rights are not living by the sword. It is simply the preservation of rights and sovereignty of will that is the right of all. Jesus on this occasion and before Pilate said legions of angels are at His disposal, in His Kingdom. God does not live by the sword.
" Further, whatever is contrary to a Divine precept is a sin. But war is contrary to a Divine precept, for it is written (Mt. 5:39): " But I say to you not to resist evil"; ... Therefore war is always sinful.
The entire passage (Mt 38-48)should be read, but here's the ending: Mt 5:44-48:
" But I tell you: Love your enemies[9] and pray for those who persecute you, that you may be sons of your Father in heaven. He causes his sun to rise on the evil and the good, and sends rain on the righteous and the unrighteous. If you love those who love you, what reward will you get? Are not even the tax collectors doing that? And if you greet only your brothers, what are you doing more than others? Do not even pagans do that? Be perfect, therefore, as your heavenly Father is perfect."
His commands are to forgive and not seek vengeance. Again it is to love others as yourself. The key to understanding this is understanding what it means to love one's self. You can not love yourself if you turn your will over to another under coercive forces. God didn't do it, and He doesn't expect anyone else to do it either. What He is asking is that those that love Him do their best to be the light of the world in His name. That is giving glory to God. Just as Jesus tossed the money changers out of His house though, He is not asking, or commanding surrender to the will of evil men. That would be contrary to loving Him, and gives glory to satin. The determination to be made by the individual here amounts to a consideration of the consequences and meaning of "turning to the other cheek".
" But war is contrary to peace. Therefore war is always a sin."
Again, war is a clash of wills that involves coercion. Peace requires the absence of coercion. As long as there is coercion war exists. The last response covers this.
Contemporary relevance:
Terrorists are waging war on the more influential western populations. What they are after is the domination of their wills for their own gain. The west has responded with a campaign to shut down terrorist operations and "educatioinal" centers. This barbarism is pure evil and should not be tolerated. Jesus wouldn't put up with this crap except for the sake of the rest of us.