This article examines whether there is a link between the legality or otherwise of an armed conflict under jus ad bellum and the subsequent conduct of the campaign under jus in bello. This is done by comparing two conflicts where the legality was not in serious dispute, The Falklands/Malvinas conflict and the Iraq War 1990-1991, and three where the legality has been questioned, Kosovo 1999, the ‘global war on terror’ and the Iraq War 2003. In looking for a common link, the author is drawn away from concerns over the jus ad bellum to doubts over the content of the relevant law governing the conduct of hostilities. Uncertainties in the law have occurred both from the extension to non-international armed conflict of ‘Hague law’, traditionally applicable only in international armed conflicts, and the overlap between human rights law and the law of armed conflict. The author concludes that there is a danger that the balance between military necessity and humanity may be disturbed so that the law will become impracticable in the cauldron of conflict to the detriment of all, soldier and civilian alike.