Go compare the number of dead. Even as absolute numbers, nevermind as percentages of the world population, the number of deaths from war from the second half of the 20th century and beyond pale in comparison to the first half.
World War II killed something like 60 million people, or about 3% of the world population. You hold up the Cold War as being bad - do you think Nukes are what made that conflict? No, they're part of what kept it from erupting into direct open warfare between NATO/the West and the Warsaw Pact/Communist Bloc. Yes, Korea was bloody (roughly about 1-2 million dead). How much more bloody would it have been had the Russians and Americans not been keen to avoid fighting one another directly lest nuclear weapons come into play? Would the USA have invaded Cuba had it not been for the threat of Nuclear War with Russia? Would Russia have invaded Western Europe at any number of points? The Cold War was unprecedented simply because there really isn't a good historical parallel of two obvious antagonists avoiding any direct conflict despite any number of flashpoints.
And why is that? Quite simply, it was that both sides knew the danger and cost of any direct conflict were far too steep and final, due to nukes.
You have a typically myopic American conservative point of view. You're picking and choosing your numbers to fit your point of view. WWII killed 60 million people? Really? So you're including the holocaust? and the flu?
And using WW2 as the "bar" for what war is like is kind of a joke. WW2 was a unique event in human history. It had never happened before and claiming that anything has prevented it from happening again is kind of a joke. It very well could happen again but now we have nukes! Yay! Do you think Germany would have had any concerns about the cost of nuclear war in the 1940s? lol
Your argument only works if all countries have free and fair democratic elections. They don't. I'd even argue that WE don't have them. When you have countries like North Korea, Russia, China, Iran with nukes? It only take a couple of dozen people to decide the cost of the war is worth it and we're all done for.