NATO's military intervention against Serbia in March 1999 underscored the tensions between state sovereignty and the defence of human rights - two principles enshrined in the UN Charter. Since the early 1990s, a consensus has developed, buttressed by UN Security Council resolutions and the practices of UN member states, that massive human-rights violations can sometimes justify encroachment on a state's sovereignty. However, this emerging consensus has not yet crystallised into clear rules establishing a right of 'humanitarian intervention', nor is it accepted by important powers such as Russia and China. Therefore, NATO nations have been at a loss to justify their Kosovo campaign in terms of international law. Rather than claiming that the Kosovo intervention is an exception, not to be repeated, Alliance members should lead a drive to adjust international law by developing clear rules for humanitarian intervention.