When is it right to fight?

No universally accepted practice currently governs the use of military force: states are going to war when they should not be, and not taking military action when they should. Available international law tools are not the problem: Chapter VII and Article 51 of the UN Charter, properly applied, between them enable the full range of necessary responses both reactive and preventive - to all likely future security threats. What is needed, in the Security Council and elsewhere, is better process to maximise the chances of reaching consensus as to when it is right to fight. Five criteria of legitimacy should be accepted as guidelines in all cases: seriousness of threat, proper purpose, last resort, proportional means and balance of consequences.