Full Idea
There is a single and only just cause for commencing a war, namely, a wrong received.
Gist of Idea
The only just cause for a war is a wrong received
Source
Francis de Vitoria (On the Law of War [1525], p.170), quoted by Michael Walzer - Just and Unjust Wars 04
Book Reference
Walzer,Michael: 'Just and Unjust Wars' [Penguin 1984], p.62
A Reaction
Walzer affirms this as one the principles of international law. In particular, mere differences of religion or politics cannot justify a war. The tricky bit is when the participants don't agree on the severity of the wrong.
Related Idea
Idea 23570 The only good reason for fighting is in defence of rights [Walzer]