Just War

Thomas Aquinas – Just War Theory from Summa Theologiae II-II, q. 40.

General Principles:

War is justified (nation A wars justly against nation B) on the following conditions:

  • A. It is called by a sovereign authority.
  • B. It has a just cause.
  • C. The combatants have morally right intentions (not vengeance or profit – see below).
  • D. Qualifying Conditions (from the theory of double-effect on his justification of killing in self-defense: ST II-II, 64, 7).
    1. Cannot intend intrinsically evil actions.
    2. A good action, or at least a morally neutral action, will have two effects: a good intended, and an evil, not intended, but tolerated.
    3. Proportionality: the good to be achieved outweighs the evil of war.


  • A. A Just War must be called by a sovereign authority. It is a nation (not individuals) who declares war.
    1. wars are just when they are in defense of the common goods.
    2. the sovereign has care for the common good (of a particular nation)
    3. when private citizens, either individually or in groups, take up arms to oppose the common good, they are not engaged in war, but in the sin of sedition and oppose the unity and peace of a people. (ST II-II, q. 42)
      a. the sin of sedition applies “not only in him who sows discord, but also in those who dissent from one another inordinately” by following the fomenters of sedition (a. 1, ad 1)
      b. the violence and discord of a people is always wrong when “it is contrary to the unity of the multitude, which is a manifest good.” (a. 2, ad 2)
      c. opposition to a tyrant can be permissible (a. 2 ad 3)
      i. “a tyrannical government is not just, because it is directed, not to the common good, but to the private good of the ruler.”
      ii. resistance to a tyrant is not permissible if “the tyrant’s rule be disturbed so inordinately, that his subjects suffer greater harm from the consequent disturbance than from the tyrant’s government” (i.e., if the violence is disproportionate to the common good lost to tyranny.)
  • B. Just Cause
    1. Thomas Aquinas addresses causes which concern the nation (nation A) itself.
      a. An enemy (nation B) is attacked because they deserve it.
      b. The enemy is guilty of some fault.
      c. A nation may war justly
      i. To avenge a wrong.
      ii. Punish enemy for refusing to make amends for some past fault.
      iii. To restore what was seized unjustly.
    2. Later thinkers have expanded the notion of just cause. (See ‘The Just War’ by Jonathan Barnes in The Cambridge History of Later Medieval Philosophy (1982), pp. 771-785.)
      a. Is war justified when someone other than the warring nation suffered from an enemy’s unjust aggression?
      i. Friends and allies: Nation A may justly war on nation B to defend nation C. (See Thomas Aquinas, ST II-II, 188, 3 ad 1)
      ii. The inhabitants of the enemy country (nation B) [a war of liberation].
      a) St. Thomas More (1535) – Yes, war may be justified for humanitarian reasons.
      b) Francisco Suarez (1617) – No, such a war violates the sovereignty of the other nation and will lead to international chaos.
      b. There has not been any actual aggression from the enemy, but nation A has reason to fear that there is a threat of an attack from nation B [a pre-emptive war].
      i. Francisco de Vitoria (1546) – No, wars are just only when redressing actual injustice.
      ii. Francis Bacon (1626) – Yes, just fear is a lawful cause for war.
      iii. Hugo Grotius (1645) – To threaten one’s neighbors is an actual injustice; it is aggression against peaceful order between nations.
  • C. The combatants in a just war must have right intentions.
    1. advancement of good and the avoidance of evil.
    2. unjust reasons include
      a. Greed.
      b. Cruelty.
      c. Vengeance.
    3. just reasons include
      a. Secure peace.
      b. Punish evil-doers.
      c. Uplifting of good.
  • D. Qualifying Conditions – from the justification of self-defense and the theory of double-effect.
    1. Cannot intend intrinsically evil actions: Combatants must respect non-combatants.
      a. Combatants who cease to be such.
      i. Surrendering.
      ii. Wounded.
      b. A nation can never justly target civilians.
      i. Civilian casualties, while foreseeable, cannot be intended.
      ii. Measures must be taken to minimize civilian casualties.
    2. The violence inherent in war is the tolerated evil secondary effect, never directly intended; the primary good effect of peace must be directly intended.
    3. Proportionality: the good to be achieved outweighs the evil of war.
      a. One cannot war justly over a slight cause.
      (i. War is a last resort.)
      (ii. There must be a reasonable hope of success; one cannot engage in justified, but hopeless actions.)
      b. One may only use the minimal force necessary to achieve just ends.

Updated 3/7/22

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