The customary international law concept of humanitarian intervention dates back to Hugo Grotius and the European politics in the 17th century. However, that customary law has been superseded by the UN Charter, which prohibits the use of force in international relations, subject to two exhaustive exceptions: UN Security Council action taken under Chapter VII, and self-defence against an armed attack. The subject of humanitarian intervention has remained a compelling foreign policy issue, especially since NATO's intervention in Kosovo in 1999, as it highlights the tension between the principle of state sovereignty – a defining pillar of the UN system and international law – and evolving international norms related to human rights and the use of force. Moreover, it has sparked normative and empirical debates over its legality, the ethics of using military force to respond to human rights violations, when it should occur, who should intervene, and whether it is effective.