Without a doubt, one of the single most influential men of the 20th century - for better or worse, mostly worse - was Vladimir Ilyich Ulyanov, alias Lenin. Arguably the Muhammad of modern times, what he started reshaped most of the world in one way or another, and killed millions along the way… Yet his effect was itself extremely improbable. Had he been arrested instead of his brother - had he died of want in Zurich - had the Germans not found him useful, or had he not proved so - he would be entirely forgotten.
prmarina posed a wide-open question: How might matters have turned out then?
It's difficult because he truly WAS so influential, and the Communist Party he created was such a diabolically effective weapon against humanity and the human spirit, that only a stub of history remains upon which to extrapolate. Between them Tsar Nicholas II and the Great War destroyed Imperial Russia; some kind of revolution was going to happen, was happening already. A relatively ineffectual Weimar / Kerensky coffee-house-intelligentsia “government” might have caused Russia per se to disintegrate into duchies, oblasts, counties, whatever, an “Imperial Germany in reverse.”
While that would still take Russia out of the war, Germany would still (presumably) lose, because that was already unavoidable barring a miracle. But the 20th century that followed, with no Communist Party - and thus no Stalin, no Nazis (“Sieg oder Bolschewismus!” The NSDAP was anti-Communist as much or more than anti-Semitic), no anti-colonial “national liberation fronts” as such… Indeed, no totalitarian New Left with all its consequences to modern society… It's hard to imagine.
This community has been dormant of late - part of Livejournal's slow decline - but this is a real stem-winder of a question and well worth turning the lights back on for!