Christian application of just war theory stems from several scriptural principles: human beings have intrinsic value (Genesis 1:27) but are also inherently sinful (Romans 3:10). God instituted human government specifically to maintain order and justice (Romans 13:1–5; 1 Peter 2:14). Mankind in general, and Christians in particular, are morally obligated to pursue a more just world (Proverbs 21:3; Micah 6:8; Matthew 5:13–16). This obligation does not, however, imply any use of violence to “advance” the faith (John 18:36). Further, God’s prohibition on killing applies to murder (Exodus 20:13), not to capital punishment (Genesis 9:6) or justified warfare (Psalm 18:34) or legitimate self-defense (Luke 22:36). At the same time, cruelty, revenge, and hatred are condemned by the Bible (Romans 12:19; Proverbs 20:22; Galatians 5:19–24).
The most commonly understood version of just war theory is grounded in these biblical ideals, simplified into five major points. Per just war theory, any conflict not meeting all of these conditions is “unjust” and morally unacceptable. It’s important to re-emphasize that this framework cannot remove all possible controversy. There will always be differences of opinion about whether a particular war—or any war—fits these points.
The five main requirements of a “just war” are as follows:
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A just war is declared by a legitimate government. According to just war theory, independent people or groups cannot act as vigilantes, taking upon themselves the right to wage aggressive warfare. This also excludes government actions that go beyond established rules; for instance, if a national leader were to ignore that country’s laws in ordering an attack or if a military leader staged a coup and immediately attacked another nation. Also, war—including the intention to attack—must be formally and officially announced before a nation engages in hostilities. This provides additional opportunities to resolve a dispute, warns civilians who might be affected, and further forces the government to validate violence beforehand.
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A just war is an act of last resort. Prior to engaging in violence, a nation must make every effort to attain its intended goals by other means. This might include diplomacy, economic or legal actions, and so forth. This is a crucial tenet of just war theory: war results from the failure of all other options. It is not one option among many. As an extension of this idea, the government should seek to end the conflict as quickly as is reasonably possible.
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A just war is fought for a just cause. The intended outcome of the war itself must be morally upright. Goals such as freeing people from death and persecution or stopping another nation’s conquest might meet this definition. A desire to gain more land or to punish another nation for some perceived insult would be an example of an unjust goal.
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A just war seeks prudent goals. Warfare is less justifiable when it has little to no chance of succeeding. This requirement is meant to balance the concept of a “just cause.” Grandiose ideas can’t be claimed as valid reasons for bloodshed. A tiny nation is foolish to launch an invasion of a military superpower—the effort is virtually guaranteed to fail, making the resulting mayhem all but pointless. Likewise, a war cannot be justified unless its goals, even noble ones, are proportionate to its toll of death and destruction. For example, the goal of improving another nation’s educational system is not a morally valid reason to engage in open warfare.
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A just war uses moral means. According to just war theory, noble ends or goals do not justify any and all actions to achieve victory. In short, the ends do not justify the means. This means a just war is restrained to proportionate levels of violence and does not engage in excessive or cruel use of force. This principle also requires making an effort to avoid civilian casualties, undue destruction, or actions that would unreasonably affect those uninvolved in the conflict. “Moral means” extends to details such as the treatment of captured soldiers and civilians and efforts to reconcile after the conflict is over.
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https://www.gotquestions.org/just-war-theory.html