I am not one of the public intellectuals who were queried by The Atlantic (link might require subscription) as to which date most changed world history — but on the Internet, you can always spout off without an invitation.
It’s hard to argue with Freeman Dyson, who nominates the day an asteroid wiped out the dinosaurs, clearing the evolutionary path for the likes of you and me.
(Actually, it’s remarkably easy to argue with Freeman Dyson. I know this, having done so over tea in Princeton, many years ago. He made it very easy indeed, despite (or perhaps because of) the fact that he was 100% right and I was 100% wrong.)
At the opposite end of the intellectual spectrum, the standup comedian W. Kamau Bell, after lamenting that there’s no way he can get this right so he might as well punt, nominates the day Michael Jackson first performed the moonwalk on national TV. Unfortunately, his intent to give the most ridiculous possible answer is thwarted by one Neera Tanden of something called the Center for American Progress, who, with an apparently straight face, nominates August 26, 1920 (the day American women gained the right to vote) — an answer that begins by placing 20th century America at the center of the Universe and proceeds downhill from there.
Other 20th-century answers (the assassination of Archduke Ferdinand, Hitler’s invasion of the Soviet Union) are at least more serious, and I think that Anne-Marie Slaughter‘s nomination of the still-very-recent-by-historical-standards signing of the Declaration of Independence on July 4, 1776 is even defensible. But then what about the Glorious Revolution of 1688, which arguably laid the political and intellectual groundwork that made the Declaration possible?
Going back a little further in that train of thought, I might nominate August 22, 1485, when the Battle of Bosworth Field brought an end to the good-government reforms of Richard III and ushered in two mostly dreadful centuries of Tudors and Stuarts — culminating in the aforementioned Glorious Revolution. Over that two-century short run, this meant a great contraction in political and economic freedom; in the long run it had a profound effect on how we think of freedom and the means by which we preserve it. Perhaps that’s too anglocentric, or at best eurocentric, an answer — but with the effects radiating far beyond Europe, I think one could make a case for it.
On the other hand, that case would certainly be trumped by the case for July 5, 1687, the day when Newton published his Principia and set modern science firmly on the path that has since transformed almost everything about the way we live (and when I say “we” I mean all humans, everywhere) and the way we understand the world (and this time I don’t mean all humans, everywhere, but I mean a very large chunk of us).
Had the Atlantic asked, that would have been my answer. What would have been yours?