2315. The accumulation of arms strikes many as a paradoxically suitable way of deterring potential adversaries from war. They see it as the most effective means of ensuring peace among nations. This method of deterrence gives rise to strong moral reservations. The arms race does not ensure peace. Far from eliminating the causes of war, it risks aggravating them. Spending enormous sums to produce ever new types of weapons impedes efforts to aid needy populations; it thwarts the development of peoples. Over-armament multiplies reasons for conflict and increases the danger of escalation.
Although a just war is based upon the natural law of self-defense, a nation that conducts a war is very different from an individual who defends himself against an aggressor. An individual, intending to defend himself, is justified if he unfortunately kills his assailant in the process. However, an individual is not justified if he takes matters into his own hands and begins to kill foreigners whom he views as enemies of his country, state, or city. The decision to go to war is reserved to the sovereign state — specifically, to those who govern the state. The state has an obligation in justice to its citizens to use physical force in order to protect or defend their individual rights. When foreigners impinge upon these rights, the state must protect its subjects. To do otherwise would deny justice to the state’s subjects. It is up to the state itself to determine when to exercise its right of coercion — subject to the limitations and considerations outlined in this article. The Church has conceded to the State the full natural right of declaring war.
There is a significant difference in context. Jesus was speaking directly to people about their personal interactions with other individuals. Just War Theory is derived from biblical principles as applied to nation-states.
However, I don’t believe you’re really willing to thoughtfully consider this answer, so I’ll ask a question: Do you believe that no person who calls himself a Christian should voluntarily participate in a war? Do you believe that no person who calls himself a Christian should violently protect himself or others from further violence?