Inter-state wars dominated the image of war so much in periods I and II that civil wars or other armed conflicts within the territories of existing states or empires were somewhat obscured. At the start of the 21st century we find ourselves in a world where armed operations are no longer essentially in the hands of governments or their authorised agents, and where the contending parties have no common characteristics, status or objectives, except the willingness to use violence. Two other characteristics of war in the 20th century stand out, the first less obviously than the second. At the same time, areas such as sub-Saharan Africa, which had been comparatively unaffected by war in period I (apart from Ethiopia, belatedly subject to colonial conquest by Italy in 1935-36), came to be theatres of armed conflict during period II, and witnessed major scenes of carnage and suffering in period III. On the other hand, during period II inter-state wars, not necessarily unconnected with the global confrontation, remained endemic in the Middle East and south Asia, and major wars directly springing from the global confrontation took place in east and south-east Asia (Korea, Indochina). Although in period III, war returned to south-east Europe, it seems very unlikely to recur in the rest of the continent.
Since 1945 inter-state wars have also disappeared from Europe, which had until then been the main battlefield region. Enemy military operations have barely touched these territories: hence the shock of the bombing of the World Trade Centre and the Pentagon on September 11. With one exception (the Chaco war of 1932-35), there were no significant inter-state wars (as distinct from civil wars) in the western hemisphere (the Americas) in the 20th century. Geographically, the impact of military operations has been highly unequal. I shall call these periods I, II and III. Chronologically, it falls into three periods: the era of world war centred on Germany (1914 to 1945), the era of confrontation between the two superpowers (1945 to 1989), and the era since the end of the classic international power system. Nevertheless, the century cannot be treated as a single block, either chronologically or geographically. The world as a whole has not been at peace since 1914, and is not at peace now. There can be no doubt, however, that the 1990s were filled with formal and informal military conflict in Europe, Africa and western and central Asia. It is a matter for debate how far the actions in which US armed forces have been involved since the end of the cold war in various parts of the globe constitute a continuation of the era of world war. This was followed, almost immediately, by some 40 years of cold war, which conformed to Hobbes's definition of war as consisting "not in battle only or the act of fighting, but in a tract of time wherein the will to contend by battle is sufficiently known". The period from 1914 to 1945 can be regarded as a single "30 years' war" interrupted only by a pause in the 1920s - between the final withdrawal of the Japanese from the Soviet Far East in 1922 and the attack on Manchuria in 1931.
It was dominated by world wars: that is to say, by wars between territorial states or alliances of states.
Taken as having begun in 1914, it was a century of almost unbroken war, with few and brief periods without organised armed conflict somewhere. The total number of deaths caused by or associated with its wars has been estimated at 187m, the equivalent of more than 10% of the world's population in 1913. The 20th century was the most murderous in recorded history.