The influence of Operation Allied Force on the development of the jus ad bellum

Some have argued that NATO's air campaign against Serbia in 1999 was manifestly unlawful, others that it was an entirely legitimate humanitarian intervention. A third position suggests that the intervention while unlawful, in the strictest sense, was nonetheless legitimate. Here, a customary law right to intervene was seen as emerging, permitting action to prevent a mass atrocity crime, even when UN Security Council authorization was absent. Did Operation Allied Force, then, add to the case for the emergence of this new customary norm? While the 1990s was a decade of humanitarian intervention, the decade since has been dominated by international action against terrorism and, of course, the effects of the highly controversial US and British led invasion of Iraq. In this context, there is scant evidence that a customary right or obligation to intervene for humanitarian reasons has crystallized since 1999. But if Kosovo achieved anything, it was to prompt greater attention to the merits of the argument in favour of a ‘responsibility to protect’. If NATO's 1999 action were repeated today in a similarly unauthorized manner it would still be unlawful, but it would perhaps be seen as a legitimate means to preventing a mass atrocity crime.