World War I was supposed to be ‘the war to end all wars’. With an estimated 9 million military deaths and up to 13 million civilian deaths caused directly and indirectly by the war, it was claimed at that point to have been the most destructive war in recorded history. Despite attempts to outlaw war at the League of Nations in 1919, however, an even more destructive war was to follow. During World War II, 1939– 45, an estimated 50– 70 million were killed in six years, with far greater numbers of civilian deaths than recorded in any previous war – about 50 per cent of all deaths. By World War II the nature of war had changed profoundly, particularly with the aerial bombardment of cities and the dropping of two atomic bombs on Japan. Above all, the Holocaust, the deliberate attempt to wipe out whole groups of people, including an estimated 6 million Jewish people, left an indelible mark on the world (Jewell et al, 2018).

In the aftermath of World War II the United Nations (UN) was established with the expressed intent of preventing future wars between nation-states and to provide an international mechanism whereby future international disputes might be resolved peacefully. The UN Charter (1945) expressed the hope ‘to save succeeding generations from the scourge of war’ and ruled against ‘wars of aggression’. It did not prohibit war however, but rather attempted to limit it to certain specific circumstances. States and, indeed, non-state groups can wage ‘war’ using the allowable criteria of ‘self-defence’, ‘defence of national interests’, ‘citizens’ protection’ or by claiming that what they are doing is not ‘war’ but dealing with an ‘emergency’, ‘criminality’ or ‘terrorism’, etc. Indeed, the use of armed conflict to advance a range of different interests has continued into the twenty-first century.