Thomas Aquinas: The Just War

Summa Theologica, Secunda Secundae, Quaestio 40, Art. 1

St. Thomas Aquinas laid the foundational framework for the Christian "Just War" theory. In this famous passage, he outlines three strict conditions that must be met for a war to be considered morally permissible: Legitimate Authority, Just Cause, and Right Intention.

1. Auctoritas Principis

Respondeo dicendum, quod ad hoc quod aliquod bellum sit iustum tria requiruntur. Primo quidem auctoritas principis, cuius mandato bellum est gerendum. Non enim pertinet ad personam privatam bellum movere: quia potest ius suum in iudicio superioris prosequi. Similiter etiam quia convocare multitudinem, quod in bellis oportet fieri, non pertinet ad privatam personam. Cum autem cura reipublicae commissa sit principibus, ad eos pertinet rempublicam civitatis, vel regni, seu provinciae sibi subditae tueri. Et sicut licite defendunt eam materiali gladio contra interiores quidem perturbatores, dum malefactores puniunt... ita etiam gladio bellico ad eos pertinet rempublicam tueri ab exterioribus hostibus.

1. The Authority of the Sovereign

For a war to be just three conditions are necessary. First, the authority of the ruler within whose competence it lies to declare war. A private individual may not declare war; for he can have recourse to the judgement of a superior to safeguard his rights. Nor has he the right to mobilize the people, which is necessary in war. But since responsibility for public affairs is entrusted to the rulers, it is they who are charged with the defence of the city, realm, or province, subject to them. And just as in the punishment of criminals they rightly defend the state against all internal disturbance with the civil arm... So also they have the duty of defending the state, with the weapons of war, against external enemies.

2. Causa Iusta

Secundo requiritur causa iusta, ut scilicet illi, qui impugnantur propter aliquam culpam, impugnationem mereantur. Unde Augustinus dicit in lib. LXXXIII q. (super Iosue qu. X): « Iusta bella solent diffiniri, quae ulciscuntur iniurias, si gens vel civitas plectenda est, quae vel vindicare neglexerit, quod a suis improbe factum est: vel reddere quod per iniuriam ablatum est ».

2. A Just Cause

Secondly, there is required a just cause: that is that those who are attacked for some offence merit such treatment. St. Augustine says (Book LXXXIII q.; Super Josue, qu. X): 'Those wars are generally defined as just which avenge some wrong, when a nation or a state is to be punished for having failed to make amends for the wrong done, or to restore what has been taken unjustly.'

3. Intentio Recta

Tertio requiritur ut sit intentio bellantium recta, qua scilicet intenditur, vel ut bonum promoveatur, vel ut malum vitetur. Unde Augustinus in lib. de Verbis Domini: « Apud veros Dei cultores etiam illa bella pacata sunt, quae non cupiditate aut crudelitate, sed pacis studio geruntur, ut mali coerceantur, et boni subleventur ». Potest autem contingere, quod etiam si sit legitima auctoritas indicentis bellum, et causa iusta, nihilominus propter pravam intentionem bellum reddatur illicitum. Dicit enim Augustinus in lib. contra Faustum (cap. LXXIV): « Nocendi cupiditas, ulciscendi crudelitas, implacatus et implacabilis animus, feritas debellandi, libido dominandi, et si quae sunt similia, haec sunt quae in bellis iure culpantur ».

3. A Right Intention

Thirdly, there is required a right intention on the part of the belligerents: either of achieving some good object or of avoiding some evil. So St. Augustine says in the book De Verbis Domini: 'For the true followers of God even wars are peaceful, not being made for greed or out of cruelty, but from desire of peace, to restrain the evil and assist the good.' So it can happen that even when war is declared by legitimate authority and there is just cause, it is, nevertheless, made unjust through evil intention. St. Augustine says in Contra Faustum (LXXIV): 'The desire to hurt, the cruelty of vendetta, the stern and implacable spirit, arrogance in victory, the thirst for power, and all that is similar, all these are justly condemned in war.'

Further Reading & Resources

Explore more about Thomas Aquinas, his political writings, and the historical development of Just War Theory from the Middle Ages to the modern era.

Stanford Encyclopedia of Philosophy

A comprehensive academic overview of the philosophical history and elements of War and Just War Theory.

Internet Encyclopedia of Philosophy

Detailed breakdown of the principles of Jus ad bellum (right to go to war) and Jus in bello (right conduct in war).

The Summa Theologica Online

Read the full context of Question 40 on War, including Aquinas's replies to objections regarding clerics and stratagems.

The School of Salamanca

Learn how 16th-century thinkers like Francisco de Vitoria adapted Aquinas's theory to early international law and the discovery of the New World.

Hugo Grotius & Laws of War

Explore the 17th-century secularization of Just War Theory by Hugo Grotius, often considered the "father of international law."

Modern Era: Michael Walzer

A look at Michael Walzer's Just and Unjust Wars, the most influential contemporary secular application of the Just War tradition.

Video Breakdown

Prefer a video summary? Watch this breakdown of Walzer's modern take on just war theory and his critique of political realism.

Watch Video Overview