What are the rules of military intervention?

France has announced it will deploy more troops to Mali to counter the Islamist insurgency there, after a night of French air raids on the rebel-held town of Diabaly.

Many people are making the comparison to Syria as the debate over whether the West should intervene in civil war there rages on. Afghanistan, Libya, Ivory Coast and the Democratic Republic of Congo are only a few of the conflicts where foreign intervention is criticised and defended. So, when, if ever, is it right to intervene?