The scale of what was at stake was made clear by Churchill when in June 1940 he sombrely explained: "If we fail, then the whole world, including the United States, including all that we have known and cared for, will sink into the abyss of a new Dark Age made more sinister, and perhaps more protracted, by the lights of a perverted science." Hitler made it clear too in 1945 when he decreed that a Germany which had failed its existential test should completely perish in an apocalypse of his own making. The idea of fleeing elsewhere to fight another day was alien to him.

Wars which fuse civil with international conflicts in a mutually escalatory fashion are notoriously vicious. They would include the disasters experienced by the Spanish, and depicted by Goya in the early 1800s, or the 1915 Turkish genocide of the Armenians, which was related to fears that they were a Russian fifth column. 

Both the Russian Civil War and the Spanish Civil War were more recent harbingers of total systemic conflicts, for the atrocities often commenced after formal hostilities had ended. The war in Spain was not only construed as a crusade against Godless heathens but, as George Orwell showed, as one against heretics on one's own side, the fate of Catholic Basques as well as Barcelona's Trotskyists. Barbarism of this kind was reflected in the German policy of deliberately starving three million or more Soviet prisoners of war to death, and the cold-blooded decision to deny "x million" more civilians food which was "subtracted" for the benefit of the Wehrmacht. 

On the other side, a society which had undergone famines and purges — and whose leaders were inured to violence by revolutionary struggle — waged war with a callous indifference to suffering: 158,000 Soviet soldiers were executed for cowardice or desertion by the NKVD, including 13,500 at Stalingrad alone. War did not halt a pervasive suspicion. Millions of repatriated PoWs — virtually all of them victims of the leadership's own military ineptitude in 1941 — were imprisoned or shot for having glimpsed the improbable sight of prosperous peasants in Romania, or the real one of houses with electric light bulbs and flush lavatories in Germany. 

Finally, throughout occupied Europe, and indeed South-East Asia, the Second World War also entailed non-uniformed combatants waging war on uniformed occupiers, sometimes with methods that might be called terrorism. It was certainly treated as such by the Germans and Japanese. The British SOE, which instigated much of this activity, took the IRA of the 1920s as a positive exemplar. 

While much of this activity is rightly celebrated, we need to acknowledge that resistance groups sometimes used torture against collaborators and spies, and that the ranks of wartime resisters included Kim Il Sung as well as the heroic cosmopolitan ladies we hear so much about. In some parts of Europe, resistance also spiralled off into vicious civil wars between rival groups of partisans, in which the Poles or Ukrainians, the Croats or Serbs, were the real enemy, rather than the Germans or Soviets. Because religion was such a marker of identity in these conflicts, it played a correspondingly large role, with clergy disgracing themselves on all sides.