Just war theory is an ethical framework used to determine when it is permissible to go to war. It originated with Catholic moral theologians like Augustine of Hippo and Thomas Aquinas, though it has had a variety of different forms over time.

Today, just war theory is divided into three categories, each with its own set of ethical principles. The categories are jus ad bellumjus in bello, and jus post bellum. These Latin terms translate roughly as ‘justice towards war’, ‘justice in war’, and ‘justice after war’.

Jus ad bellum

When political leaders are trying to decide whether to go to war or not, just war theory requires them to test their decision by applying several principles:

  • Is it for a just cause?

This requires war only be used in response to serious wrongs. The most common example of just cause is self-defence, though coming to the defence of another innocent nation is also seen as a just cause by many (and perhaps the highest cause).

  • Is it with the right intention?

This requires that war-time political leaders be solely motivated, at a personal level, by reasons that make a war just. For example, even if war is waged in defence of another innocent country, leaders cannot resort to war because it will assist their re-election campaign.

  • Is it from a legitimate authority?

This demands war only be declared by leaders of a recognised political community and with the political requirements of that community.

  • Does it have due proportionality?

This requires us to imagine what the world would look like if we either did or didn’t go to war. For a war to be ‘just’ the quality of the peace resulting from war needs to superior to what would have happened if no war had been fought. This also requires we have some probability of success in going to war – otherwise people will suffer and die needlessly.

  • Is it the last resort?

This says we should explore all other reasonable options before going to war – negotiation, diplomacy, economic sanctions and so on.

Even if the principles of jus ad bellum are met, there are still ways a war can be unjust.

Jus in bello

These are the ethical principles that govern the way combatants conduct themselves in the ‘theatre of war’.

  • Discrimination requires combatants only to attack legitimate targets. Civilians, medics and aid workers, for example, cannot be the deliberate targets of military attack. However, according the principle of double-effect, military attacks that kill some civilians as a side-effect may be permissible if they are both necessary and proportionate.
  • Proportionality applies to both jus ad bellum and jus in bello. Jus in bello requires that in a particular operation, combatants do not use force or cause harm that exceeds strategic or ethical benefits. The general idea is that you should use the minimum amount of force necessary to achieve legitimate military aims and objectives.
  • No intrinsically unethical means is a debated principle in just war theory. Some theorists believe there are actions which are always unjustified, whether or not they are used against enemy combatants or are proportionate to our goals. Torture, shooting to maim and biological weapons are commonly-used examples.
  • ‘Following orders’ is not a defence as the war crime tribunals after the Second World War clearly established. Military personnel may not be legally or ethically excused for following illegal or unethical orders. Every person bearing arms is responsible for their conduct – not just their commanders.

Jus post bello

Once a war is completed, steps are necessary to transition from a state of war to a state of peace. Jus post bello is a new area of just war theory aimed at identifying principles for this period. Some of the principles that have been suggested (though there isn’t much consensus yet) are:

  • Status quo ante bellum, a Latin term meaning ‘the way things were before war’ – basically rights, property and borders should be restored to how they were before war broke out. Some suggest this is a problem because those can be the exact conditions which led to war in the first place.
  • Punishment for war crimes is a crucial step to re-installing a just system of governance. From political leaders down to combatants, any serious offences on either side of the conflict need to be brought to justice.
  • Compensation of victims suggests that, as much as possible, the innocent victims of conflict be compensated for their losses (though some of the harms of war will be almost impossible to adequately compensate, such as the loss of family members).
  • Peace treaties need to be fair and just to all parties, including those who are guilty for the war occurring.

Just war theory provides the basis for exercising ‘ethical restraint’ in war. Without restraint, philosopher Michael Ignatieff, argues there is no way to tell the difference between a ‘warrior’ and a ‘barbarian’.