We would probably have gone to war with Japan in any case if for no other reason than they attacked us. Japan has had designs on a modern empire since 1900 and were emboldened by their destruction of the Russian Naval Fleet in 1905 and their subsequent colonization of Korea and Manchuria.
There is no basis to argue that the toothless talking shop inaccurately called "The League of Nations" would have had any influence over Japanese ambitions in Asia if we had been involved.
The existence of Austria-Hungary Empire provided no stabilization to prevent The Seven Years War (the first actual World War in global history) or any other of the dozen or so European conflicts that occurred in the intervening years.
Germany was a violent, aggressive neighbor long before the unification of the German States in 1871, having attacked and conquered Austria, France and Balkan States in the years prior.
I submit there is nothing Americans can do to prevent Europeans from killing each other that wouldn't be more damaging to our own country than just staying out of their shenanigans.