The doctrine defining a just war goes back to Augustine. Augustine was a bishop at a time when the Roman Empire had adopted Christianity as its official religion. Therefore, the Caesar was a Christian, and the question quite naturally arose as to when a Christian emperor could command troops into the battlefield.
This was also a time when Northern European tribes (“barbarians”) were invading Rome. Rome had to either wage war or turn the Empire over the barbarians.
The Wikipedia summarizes his views simply —
Augustine developed a theology of just war, that is, war that is acceptable under certain conditions. First, war must occur for a good and just purpose rather than for self-gain or as an exercise of power. Second, just war must be waged by a properly instituted authority such as the state. Third, peace must be a central motive even in the midst of violence.
Since that time, many theologians have sought to refine the theory. There’s an excellent article by Arthur F. Holmes that explains the theory. Speaking of Augustine, he writes,
He tells the Roman General Boniface, who was later to defend Carthage against the Vandals, that war is not a matter of choice but of necessity, forced on us by the need to control violence in a fallen world. It is waged only to restore peace, so he should preserve the spirit of a peacemaker, limiting violence to what is needed in resisting and deterring aggression, and extending mercy to the vanquished and the captive.
Holmes offer some further history of the teaching —
The sixteenth-century Spanish theologian Francisco de Vitoria develops the theory further. Examining King Philip’s wars against the American Indians, he condemns their lack of just cause. War, he insists, is not justified for religious reasons (to convert the heathen) nor for economic causes (to gain their gold) nor for political reasons (to extend the empire). The Indians, however pagan, immoral and uncivilized, are human beings with rights equal to those of all other persons. The natural law protects them against violence and injustice.
Vitoria also asks whether the soldier who doubts the justice of a cause should fight. Ordinarily, one should trust the lawful government to do what is lawful. But if justice is seriously in doubt, and if careful inquiry does not allay those doubts, then the soldier should refuse to fight. Selective conscientious objection is the corollary of a just war ethic.
The Protestant Reformers meantime addressed the problem in similar terms. … The use of the sword, [Luther] argues, is divinely entrusted to governments in order to repel injustice and keep the peace. It can, therefore, be a work of love for the common good. But only defensive war is just, including action to recover unjustly seized property from previous conflicts. This rules out religious wars, aggression and any attempt to revenge an insult. Only the highest governmental authority has the right to initiate military action, so that rebellion is always unjustified. It was on this basis that he opposed the famous Peasants’ Revolt. Yet the ruler, if wrong, should be disobeyed: selective conscientious disobedience is not revolt.
He builds his case from Rom 13 as well as —
(1 Pet 2:13-14) Submit yourselves for the Lord’s sake to every authority instituted among men: whether to the king, as the supreme authority, 14 or to governors, who are sent by him to punish those who do wrong and to commend those who do right.
He summarizes the Old Testament record thusly —
These [New Testament] limitations are reinforced when one considers the Old Testament attitude toward war. While military conflict is regarded as a tragic fact of life, one for which God strengthens his people and one which God uses in the execution of justice, it is nonetheless lamented as an evil from whose scourge humanity must be delivered. Israel was instructed to limit the destruction and violence involved in its conquest of Canaan (Deut. 2). David was not allowed to build God’s temple because he was a man of war (1 Chron. 22:8-9; 28:3). The psalmist grieved over violence, looking to the God who makes war cease and destroys its weaponry (Ps. 46; 120). The prophets condemned its fratricide and its atrocities (for example Amos 1?2), mourned its destruction (Lam.), and gloried in the One who will finally bring peace and justice to earth so that none need even feel afraid (Is. 2:1-5; 9:1-7; 11:1-9).
Holmes explains the overarching goal of the theory —
Third, the just war theory does not try to justify war. Rather it tries to bring war under the control of justice so that, if consistently practiced by all parties to a dispute, it would eliminate war altogether. It insists that the only just cause for going to war is defense against aggression. If all parties adhered to this rule, then nobody would ever be an aggressor and no war would ever occur. The basic intention of the just war theory, then, is to condemn war and to prevent it by moral persuasion. But since people will sometimes not be so persuaded, it proceeds to limit war – its occasion, its goals, its weaponry and methods – so as to reduce the evils that have not been altogether prevented.
He offers these guiding principles as summarizing the thought of current just war theologians —
- Just cause. All aggression is condemned; only defensive war is legitimate.
- Just intention. The only legitimate intention is to secure a just peace for all involved. Neither revenge nor conquest nor economic gain nor ideological supremacy are justified.
- Last resort. War may only be entered upon when all negotiations and compromise have been tried and failed.
- Formal declaration. Since the use of military force is the prerogative of governments, not of private individuals, a state of war must be officially declared by the highest authorities.
- Limited objectives. If the purpose is peace, then unconditional surrender or the destruction of a nation’s economic or political institutions is an unwarranted objective.
- Proportionate means. The weaponry and the force used should be limited to what is needed to repel the aggression and deter future attacks, that is to say to secure a just peace. Total or unlimited war is ruled out.
- Noncombatant immunity. Since war is an official act of government, only those who are officially agents of government may fight, and individuals not actively contributing to the conflict including POW’s and [c]asualties as well as civilian nonparticipants) should be immune from attack.
Now, each of these points raises a host of subsidiary questions. For example, under 3, how many people must suffer or die while the diplomats dither? Obviously, evil rulers are happy to use the diplomats to delay war while they consolidate their forces and continue their evil. And under 5, what if the nation’s political institutions are evil? Was it wrong for the Allies to oust the Nazis from control of Germany? Should they have stopped at the borders of the country? Should the US have forced a change in government in Japan?
So I’m not entirely sold on this version of the theory, but I have to agree from a broad perspective. Yes, the scriptures give the government the power to defend its people from evil, and this fact leads to a limitation on what wars are truly just.