Its just a buzzword, a talking point parroted by american apologists. Never existed in the past and it certainly doesnt exist now. "Rules based international order" in practice means: NATO gets to do whatever the shit they want, as they have always done. That's it. That's the whole rules in actual practice.
And regarding the supposed reduction in warfare after WWII:
You can thank the existence of the Soviet Union, which united much of the western imperialist powers (Europe, America and its vessel states like Japan, Australia or South Korea) against the first real menace against capitalism in history. That's pretty much the reason why inter-european warfare stopped after WWII. And they focused on warfare against the third world (Korea, Vietnam, Indonesia, South America, Central America, etc.) They function as an imperialist block with the same interests. It has little to do with the supposed existence of "rules" or an imaginary "order".