This lecture by Professor Nigel Biggar was delivered at Policy Exchange on 31 January 2017, with a Vote of Thanks from Lieutenant General Mark Carleton-Smith CBE, Deputy Chief of the Defence Staff (Military Strategy and Operations), on behalf of the Chief of the Defence Staff, Air Chief Marshal Sir Stuart Peach GBE KCB. It was also published jointly by Policy Exchange’s Britain in the World project and the McDonald Centre, University of Oxford. The lecture reflects on some of the moral lessons we should and should not learn from the recent history of British military interventions abroad, in view of the challenges and dilemmas Britain is likely continue to face in the future. After Iraq, it asks, what are the circumstances in which Britain should go to war?