1942: Europe from Brittany to the Ukraine, the Mediterranean to the Arctic Circle in Hitler's hands. Bombs falling on London. Balkan states and puppet regimes aligned with Hitler with the potential for Spain, Turkey and other countries to throw in with him as well. The Pacific Hemisphere from Thailand to the Aleutians, Manchuria to New Guinea under Japanese control. The potential for millions of colonial peoples to joint with the Axis against the US and UK.
1950: The millions of Russia and China, North Korea and Eastern Europe taught to revere Stalin. Northern Eurasia from Stettin and Vienna to Shanghai and Vladivostok all apparently under Red control. The largest armies in the world, war-tested, equipped with nuclear weapons, and indoctrinated to view us as the enemy. Communist sympathizers in France and Italy and the rest of Europe. The potential for Indians and Africans and Arabs and Latin Americans to fall under Communist rule.
The prospect was certainly darker then. What Prager's doing is contrasting a potential situation today with what he takes to be the actual situation then. What he forgets is the potential for trouble then. Prager ought to compare potential with potential or actual with actual (so far as we can get at the actual), not potential now with actual then. The actual destruction and suffering were greater then than now, and the outlook for the future was very dark indeed in those days.
Government power counts for a lot. Hitler or Mussolini, Stalin or Mao could send tens of millions into combat. I don't want to minimize the threats and difficulties today. Terrorists can do a lot of damage with an airliner or a truck filled with explosives or a nuclear weapon. But it helps to keep things in perspective.
Prager's at his worst, though, when he says that the totalitarians of the Thirties, Forties, and Fifties wanted to live, while Islamic terrorists today want to die. Listen to the songs or watch the films of Nazi Germany or militarist Japan: they expressed the same passion to die, in this case for nation or leader. It's funny that we forget how strong the idea of a "glorious death" for the fatherland was in recent Western history.
I think the fair comparison is to 1935-6. Nazis already showed what they want, but no large scale blood-letting yet. Spain is bleeding, but its mostly reds v browns, so who cares. Bolsheviks won the very bloody Civil War in 1922 and after some years of relative quite exterminated productive farmers as class and further subjugated Ukraine with the famine. But it was "far away" so the west is still full of utopian communist sympathizers. Japan is rising, but their demands for more resources are understandable, aren't they?
We are not in 1942 yet, and that's the point: this way around we can avoid it.