The use of force and the regulation of armed conflict in the 21st century raises new and challenging questions in international law and policy. This timely study brings together leading scholars – including ethicists, political scientists and international lawyers – to address the use of force, beginning with NATO’s 1999 intervention in Kosovo up to the US-led invasion of Iraq. In some important respects, the legal regime for force regulation – in place since the adoption of the UN Charter in 1945 – provides a normative restraint on international relations, but is it wholly relevant and effective with regard to contemporary events? The contributors investigate what options are or should be lawful in the 21st century.