Modern just war thinking gathers the older ideas into a list of tests for going to war. Different writers word them differently, but most lists include six: just cause, legitimate authority, right intention, last resort, a reasonable probability of success and proportionality. Some add a seventh, public declaration. The tests are meant to be used together. Passing one does not make a war just; failing any one is a serious warning.

Just cause and legitimate authority

Medieval writers accepted several just causes, including recovering stolen property and punishing wrongdoing. Modern thought has narrowed the list sharply. Today the core just cause is resisting aggression, an armed attack on a state's territory, independence or people, whether the victim is oneself or another. Walzer called this framework the legalist paradigm, because it treats states somewhat like citizens in a society, with aggression as the central crime one state can commit against another.

Legitimate authority asks who has the right to take a community to war. For Aquinas the answer was a sovereign ruler. Today it usually means a state acting through its lawful procedures and, in international law, the UN Security Council, since the UN Charter permits force only in self-defense or with the Council's approval. The test raises hard cases. Can rebels against a tyrant, or a people fighting colonial rule, ever have legitimate authority? Many thinkers now say yes, if they genuinely represent the people they claim to fight for.

Right intention

Right intention requires that the just cause, not hidden gain, drive the war. A state that defends an ally in order to seize its oil fields fails this test even if the ally really was attacked. Real decisions almost always involve mixed motives, and most writers accept that. What they reject is a just cause used as a cover story, so that the war would have been fought anyway and would continue even once the just aim was achieved.

Last resort and probability of success

Last resort means that reasonable peaceful options have been tried or are plainly hopeless. It cannot mean that every conceivable alternative has been exhausted, because one more meeting or one more round of sanctions is always possible, and an aggressor could exploit endless delay. The judgment is about whether waiting longer is likely to achieve the just aim at lower cost.

The test of probability of success forbids hopeless wars that waste lives, sending soldiers to die for a cause that cannot be won. It is a humane test, but it has a troubling edge. Critics say the success test may deny weak nations the right to resist: a small country invaded by a giant might lose, yet resisting could preserve its dignity, buy time for allies or make the conquest too costly to keep. Many writers therefore treat the test as a strong caution rather than an absolute bar.

Proportionality

Proportionality for going to war requires that the war's expected good outweigh its expected harms, counting the enemy's civilians too. A border incident that killed a dozen people does not justify a war likely to kill tens of thousands. This test looks forward and is full of uncertainty, since wars rarely go as planned.

Are the tests too elastic?

A common objection is that every government claims its wars pass every test, so the list justifies everything. Defenders answer that the criteria force leaders to justify war publicly in shared terms, terms that journalists, courts, opposition parties and citizens can then test against the evidence. A leader who must explain why war is the last resort can be asked what alternatives were tried. Those questions are not useless just because some leaders lie in answering them.

Worked example: Kuwait, 1990 to 1991

On 2 August 1990 Iraq invaded Kuwait and soon declared it annexed. The UN Security Council demanded immediate withdrawal, imposed broad economic sanctions within days, and in November authorized member states to use all necessary means to expel Iraq if it had not left by 15 January 1991. An air campaign by a coalition of more than thirty countries began on 17 January. A ground offensive launched on 24 February ended with a ceasefire on 28 February.

Run the tests. Just cause looks strong: a sovereign state had been swallowed by its neighbor. Legitimate authority was unusually clear, since the Security Council had approved force. Last resort was disputed. Some critics, including two former chairmen of the US Joint Chiefs of Staff who testified to Congress in late 1990, urged giving sanctions more time to work. Right intention was questioned too: oil supplies and regional power were obviously at stake. Defenders replied that mixed motives do not cancel a just cause, and the coalition did stop once Kuwait was freed rather than marching on Baghdad. Proportionality is debated still. The war was short and the probability of success was high, but thousands of Iraqi soldiers and civilians died, and damage to Iraq's power grid caused suffering long after the fighting. Many just war thinkers count this among the clearer cases of a justified war, yet even here honest people reach different verdicts on particular tests.