Donald Sensing has a counterfactual arguing that all of the 20th century's bloody woes can be traced to the actions of a couple hundred Paris taxicabs in September of 1914. It's an interesting setup, and it does what counterfactuals are supposed to do - demonstrates an argument about the interplay of causation in a particular sequence of events. His argument - that the excision of all of those bad political effects would have resulted in a much less bloody century - isn't one I find particularly convincing, but that's because he doesn't extend the line of causation in any direction. A short World War (really, more of a brief, oversize repeat of the Franco-Prussian War) wouldn't have caused the exact chain of events, of course. I find his short-term series of events convincing.
But I have to wonder if our hypothetical survivors of the 1914 war mightn't have come back for a second try a few years later. Additionally, one has to wonder if a second French defeat mightn't have brought on the proletarian revolution in France. The Ottoman Empire had been rotting for the better part of a century. The first half of the century wouldn't have closed without a fall of some sort. Fascism was a pre-war heresy of the left; it wouldn't have enjoyed the overwhelming advantages of the great cataclysm, but what happens in an unstable Europe of limited, long, brutal wars? The Liberal order was due for a collapse - the "Peace of Dives" was a tinderbox. Even if the initial brushfire had been contained in 1914, I have to wonder if later, now-obscure lightning strikes might not have lit the remaining brush unburnt by our hypothetical.
In the end, my hypotheticals reflect my personal prejudices. Historical momentum tends to be conserved. Actions have chaotic, but converging consequences. "Everything that rises...must converge."