casus belli
In classical international law a casus belli was a grievance (an insult, an attack on nationals, a breach of treaty) treated as a sufficient reason to start a war. War was then a lawful instrument of policy.
The UN Charter changed this. Article 2(4) prohibits the threat or use of force against another state, and the only accepted exceptions are self-defence against an armed attack (Article 51) and action authorised by the Security Council under Chapter VII. A grievance, however serious, no longer gives a lawful right to wage war; the question now falls under jus ad bellum. Do not confuse it with casus foederis, the event that triggers an obligation under a treaty of alliance.
Example
Even a proved border provocation is not a lawful casus belli today unless it amounts to an armed attack justifying self-defence under Article 51 of the Charter.
In Indian law
- Article 2(4), Charter of the United Nations (prohibition of the use of force)
- Article 51, Charter of the United Nations (inherent right of self-defence against an armed attack)