Thomas Aquinas, adopting the base theory of St Augustine and developing it, came up with the 7 principles of Jus ad Bellum (Latin for Right to War) and the 2 principles of Jus in Bello (Translated as, The Law in Waging War).

7 Principles of Jus ad Bellum:

  1. War must be fought for a just cause – to save life or protect human rights; to secure justice, remedy injustice; it must be defensive not aggressive.
  2. War must be declared by a competent authority. In most cases, the government would be the legitimate authority to declare war.
  3. There must be comparison of justice on both sides. This is, of course, difficult to achieve, since both sides will inevitably maintain that they are fighting for a just cause.
  4. There must be right intention, which must be as just as its cause.
  5. It must be a last resort, after all negotiation, arbitration and non-military sanctions have failed.
  6. There should be a reasonable proportion of success, so that it’s outcome results in a better states of affairs.
  7. There should be a reasonable proportion between the injustice being fought and the suffering likely to be inflicted by war. The cause of justice must not be caused by unjust means, which includes inflicting harm on those you are aiming to protect.

2 Principles of Jus in Bello:

  1. Proportionality must be exercised, i.e weapons used should be proportional to the threat and minimum force should be used.
  2. Warfare must be discriminate – civilians should be protected and not directly targeted.  The use of nuclear weapons renders discrimination impossible.