I really don't think that's objectively true. The "relative peace" is quite relative after all, depending a lot on where you live, and at best it's less than a century old (75 years since the end of WWII). I think there have been many other societies that have passed an occasional century or so in relative peace throughout history. It's just that the wars are more prominent in the history books than the boring peaceful years between them.
I'm not saying we haven't achieved anything... wide-spread recognition of basic human rights, near abolishment of slavery, valuing democracy and self-determination have all been taking to new heights over the last few generations. But at the moment all this is looking pretty fragile.
And personally, RBG's death today left me feeling more than ever that we're on a knife's edge, and that we could fall right back into those millennia-old patterns that are only occasionally interrupted by a century or so of "relative peace". Or worse.
Classical liberalism/the open society/the Enlightenment all came about centuries before WWII. There were wars (WWII included) fought against people who explicitly rejected these principles, but coming up with those principles in the first place was the part I was referring to.
