Alternate histories are a favorite recurrent topic here at the SDMB. One of the more recent ones discusses What if the Roman Empire had collapsed in the 2nd century?, two centuries before it actually did. Aside from the obvious history points such as the Battle of Gettysburg, what are some of the more obscure points where you believe history could have changed radically?
One of my favorites is the assassination of Tsar Alexander II in 1881. In one of history’s bitterest ironies, radicals killed the most progressive reformist Tsar to reign in over a hundred years. Alexander’s murder set the stage for his successors to adopt hard-line reactionary policies that all but guaranteed eventual revolution.
What if Kaiser William had been smothered in his crib? (Or killed by Annie Oakley when (as part of Wild Bill Hicock’s Wild West Show) she shot the cigar out of his mouth.
What is Edward the whatever number had not abdicated?
What is King Leopold of Belgium had screwed up the Congo so publicly that (almost anything but out time line) happened there?
What if the American Indians in the Transmississippi had had a Gengis Khan to unite them in c. 1790?
What if European diseases wiped out a huge hunk of the African population in (pick a year)?
What if say, zebras were as easy to domesticate as a horse ? Or if the ancient ancestors of the Native Americans thought of domesticating the native horses before they killed them off ? Having a horse or equivalent would have been a major advantage for the development of those regions.
I’m not sure what the question is. Are you asking a "What if…"question, ie. the change in what historical event would have resulted in the most change later on (which is what the first couple of responders seem to be answering) or are you asking what actual historical event little known to most people had the most profound impact on today’s world?
What if the Jacobites had decided to keep going on to London, instead tehy reached Derby, had an argument, turned back around and went home to Soctland.
No what ifs. Two turning points in history which most people have never heard of and which profoundly affected the world (at least the Western world) are:
The Battle of Manzikert (1071) in which the Seljuk Turks crushed the Byzantine Army, gained control of Anatolia, and began rapid growth of what was to become the Turkish Empire, and
The death of the Great Khan Ogedei in 1242. The Mongol armies had swept west and devastated Russia, Poland, and Hungary, defeating the best armies the West was able to raise against them. The Mongols were resting up on the plains of Hungary preparing to continue their invasion of Central and Western Europe when they received word that the Great Khan had died in Mongolia. The Mongol armies withdrew to the Russian steppes so that their leaders could return to Mongolia to take part in the selection of the next Khan who, as it turned out, decided to forget about Europe and concentrate on China and the Middle East. The Mongols army never returned to Europe. (Except for a contingent of Mongol troops the Russians used to quash the Hungarian Uprising of 1956, the first time in more than 700 years that the Mongols trod the streets of Budapest.)
During the 15th Century, if China had continued its naval exploration, rather than suddenly stopping and becoming an insular society, things would be very different.
Not a turning point more of a period, but if the 30 years war (and the plague following it) wouldn’t have happend, then the expansion of the German knights eastward and the settling of those forested areas would have continued. (Whether this would have turned out good is a different question).
But because the war and the diseases and hunger following it wiped out 90% of the population in Mid-Germany, the expansion was stopped, and internal development slowed.
I’ve heard it said that a major driver for the ( relative ) elimination of slavery was the invention of the horse collar; it allowed a horse to do far more work and made slaves less profitable for much of the mindless labor they were used for. A horse collar is a simple enough idea that it could have been invented far earlier than it was. Alone it wouldn’t have eliminated slavery I think, but by making it less useful it would have cut back the influence of slavery in society, and contributed to there being less of and and it ending sooner.