As mentioned earlier, this theory originated with St. Augustine. Good read:

"Our Lord's driving out the money changers from the Temple is used to justify war by the righteous. Those against war claim that "there is no evidence of violence done to people. . . Christ's action does not show the force of war; rather the scourge is used to show His messianic role."(21) When Our Lord refers to no greater love than when one lays down one's life for a friend pro-war advocates use it to praise soldiers giving their life for their country. But to use the text in this way is to extract it from a call to love. "The purpose of the soldier is not to lay down his life, but to make the enemy lay down his."(22) The text of St. Paul that everyone must be subject to higher authorities is used to justify the State's right to require its citizens to kill in war. (cf. Rom. 13:1-7) This passage is seen by those against war to be pointing to the legitimate realm of police power in civil governments. Jean Lasserre and Richard McSorley, S.J., examine and explain the distinction between "indiscriminate violence of war and the legitimate function of the police who seek to contain violence and uphold the law."(23) The same authors "address the notion of violence done in the spirit of love and both reject it on scriptural grounds."(24) Jesus as our model rejected physical force and revenge that "led inevitably to His Passion and humiliating death on the Cross."(25)

St. Paul's writings reflect the spirit of the Gospel being lived by the Christian communities. He speaks of not rendering "evil for evil" but of overcoming it with good. (Rom. 12:17,21) The Martyrs of the pre-Constantine era are an example of Christian praxis in the face of violent opposition. It is "'the story of the successful Christian revolution against the Roman Empire,'" that is, nonviolence over violence.(26)

Regarding Christian military service "before the year 172 there was none. . .Evidence of Christians serving in the Roman army before the third century is suspect. . . A study of tombstone inscriptions reveals that only 7 Christians out of 4,700 extant inscriptions were members of the military."(27) This evidence shows few served in the military. Of the 7 military tombstone inscriptions it is not clear whether these people converted in the service or entered already converted. In 298 Marcellus, a centurion, refused to continue military service. In front of the Emperor he claimed he would only serve Jesus Christ. Brought to trial Marcellus argued that "he could not inflict wounds."(2 He was executed for defying the Emperor. In the third century "Christians certainly served in the army; but their numbers were small, their service peaceful, and their testimony one of peace and not of violence."(29)

When we arrive at St. Augustine his justifications for war "are based on Cicero and other Roman thinkers. . . (with) Manichaean and Neoplatonic influences."(30) There is a dualism in his writings over the possibility "to love an enemy internally and still to kill him."(31) St. Augustine "seems never to have satisfied himself as to how it (war) is fully compatible with Christian charity."(32)

During the Middle Ages while the Just War Theory was becoming more developed and Crusades were being preached peace movements among Christians flourished, also. St. Francis "may not have been a pacifist, but 'he lived as if he were.'"(33) A peace movement known as the "Great Alleluia" involving thousands of people took place in northern Italy in the 13th century. In 1233 the movement had grown to such proportions that 400,000 people gathered to demonstrate for peace and reconciliation. Another Italian peace movement known as the "Bianchi" moved about in thousands from city to city. Peacemaking was their major work. One chronicler notes that by the time one of these processions reached Rome its numbers had swelled to 200,000.

Another approach to condemnation of war that began to develop at this time was a distinction between war in the abstract and the reality of war. These thinkers say that while one can hold to war as justified in theory, in practice it cannot be justified. In the 16th century St. Thomas More was such a thinker. In his "Utopia," or ideal society, St. Thomas allowed for war only as a defensive measure. But "in reality no war that he knew in history, in the present, or in the foreseeable future was just."(34) So St. Thomas More used the Just War in theory to condemn all wars in reality.

Some modern authors use the same approach. Fr. Emmanuel McCarthy says that he has "found no war that has ever been fought according to the moral norms of the Just War Theory. . .(and) cannot imagine a war that could be conducted within its moral perimeters."(35)

To sum up what lies behind many Pacifist claims is this: "war's central action of inflicting suffering and death is directly opposed to the example of Christ in enduring these same realities. . .(it) conflicts with the essence of the Gospel."(36) The Just War Theory is looked upon by them as a compromise to Christian ethics introduced during the Constantine era when the Church became identified with the Roman Empire. These are some of the arguments presented by Pacifists but in no way is it an exhaustive list...."

source:
http://www.monksofadoration.org/justwar.html

The bold is what this baby-killer used to justify serving. I guess its relly the only reason I pay taxes also.