WW1 was the Central Powers' to lose from the very beginning, but the Entente came very close to collapse at many times as well. Given that in purely military terms the two sides were quite evenly matched on the Western Front it's not inconcieveable that given certain PoDs, (e.g. no American entry, earlier peace in the East, or even a scenario similar to the one in @TheReformer's To the Victors Go the Spoilers) they could have continued to go at it for another year or two. By 1919 or 1920 (for instance), the economic strains would lead to revolution on both sides, at the very least in France and Italy for the Entente, while all of the Central Powers would probably collapse. Add in some other PoDs involving the Bolsheviks (e.g. better relationship between Bolsheviks and their erstwhile/potential allies in the (Left) SRs, Menshevik (Internationalists), and anarchists, similar to Saving Soviet Democracy) leaving them in a better position when it comes to the Russian Civil War and with more resources to dedicate to Western Europe, and you might be seeing a very different Europe from OTL, and a more powerful and diverse, if not necessarily powerful left-wing (I could see it going both ways). Obviously, it's possible that things take a right turn instead, but my understanding is that nationalism was highly discredited immediately following the war and only grew in the aftermath of Versailles.