I had an oddball thought as I was falling asleep. Nuclear Weapons may be the only thing that maintains the amount of peace we actually have.
First, let's review history:
I'm actually going to start with the animal kingdom. As far as I know all carnivores are territorial, marking and fighting to defend/capture territory. This extends even to omnivores, and for this example I'm going to cite the Gombe Chimpanzee War, in which a tribe of Chimps murdered everyone in a neighboring territory and then were driven out themselves by an even stronger tribe.
Looking at the Paleolithic Era up to the Bronze age, this Wikipedia article lays out a quick description of warfare from before the invention of society through the Iron Age! What I find really interesting, and relevant, is that there was relative peace in the Paleolithic era because "The development of the throwing-spear, together with ambush hunting techniques, made potential violence between hunting parties very costly, dictating cooperation and maintenance of low population densities to prevent competition for resources."
War seems to have kicked up again with the advent of sedentism, that is essentially settling in one place rather than the constant wandering of prior hunter-gatherer tribes.
War among small tribes and city-states is actually rampant throughout history. If you look at China's Warring States Period once the Zhou Dynasty began to collapse into over a hundred smaller states which immediately began to war with each other.
Japan went through a period of warfare so similar during their Sengoku Period that they later stole the phrase "Warring States" from China.
Lords serving under the same King in Medieval Europe would often battle each other, Scottish Clans frequently fought each other, and the Icelandic Commonwealth, often brought up by Libertarians as an example of a working government-free society, was rife with blood-feuds and in the Sturlung Era powerful chieftains warred with each other until the Commonwealth collapsed.
There are, of course, plenty of examples throughout history of much larger scale warfare and conquering from Sargon of Akkad to Alexander the Great to Julius Caesar, Attila the Hun, Napoleon, and Hitler.
Hitler... yes, World War II, at the end of which the United States dropped the first ever atomic bombs in Japan.
Of course there is still war, but it seems vastly different now. What could have been a long, bloody war over ideology between the US and the USSR became a cold war. Germany waited and negotiated for reunification. No one in countries with nuclear weapons or allied with them seems likely to start a war or try to conquer territory. The outlier here is Russia taking over the Ukraine, but even that was a non-event.
What we see since the advent of nuclear powers is proxy wars, such as Vietnam, or attempts to meddle in the politics of a region, inciting warfare there or installing rulers. Essentially what is happening now mostly seems to be nuclear superpowers vying for power without actually fighting each other.
The only recent "conqueror" to arise since WWII has been ISIS, which as terrible as they were arose in regions without nuclear weapons and had none themselves.
I'm hesitant to lay this entirely at the feet of nuclear weapons, since at any point they could be used. One might wonder, however, if the US would have so eagerly invaded Iraq in 2003 if Saddam had ICBMs capable of destroying American cities. With the converse point being, if Saddam had nuclear weapons would he have used them in his genocide of the Marsh Arabs or in conquering other territories.
Largely it takes me back to the Paleolitic era quote I mentioned earlier. To paraphrase:
"The development of nuclear weapons made potential violence between countries very costly, dictating cooperation..."
Have we lessened our warlike tendencies because we're more civilized? Or because the potential cost of violence has gotten to high?
