I’ll go with: The appointment of Adolf Hitler to Chancellor of Germany by President Hindenburg.
People forget that Hitler came to power via appointment, not election, not revolution. A bunch of conservatives - who thought they could control Hitler - talked Hindenberg into appointing Hitler. And that led directly to WW2, and many things that occurred afterwards.
I’ll go with the invention of semiconductors in 1949. Wars come and go; this technology changed everything.
Sure but… and then, what ? It didn’t really teach us anything new or change anything for the people on the ground. It was essentially a hybrid dick waving contest slash threat of total annihilation.
Me, I’d nominate the Missile Crisis - because that’s probably the tipping point that let even the maddest assholes in charge realize that maybe, just mayyyybe permanent arms race and rigid ultrajingoistic fuckery should possibly maybe stop somewhere and both superpowers suddenly started opening up inroads towards détente and disarmament (albeit with a temporary stop at “a whole bunch of brutal proxy wars” first).
Hitler coming to power was a lot like Vanilla Ice dominating the music charts. If he hadn’t done it, inevitably someone very similar to him would have. World War II was inevitable when the gratuitously harsh conditions of the Treaty of Versailles were forced on Germany two decades earlier.
The atomic bomb is a good answer. It was unprecedented in a way that a Russian revolution and an assassinated archduke really weren’t.
My answer is, arming airplanes in World War I. Airplanes were wood-and-canvas novelties up until that point, and their initial military use was for reconnaissance. The first pilots from opposing sides actually waved at each other and went about their business, until one day one of them was issued a revolver and told to shoot the other one. Mounting machine guns, figuring out how to shoot through propellers without damaging them, dropping a bomb from a plane, all this happened very quickly after the first time a pilot was issued a revolver.
Agreed, it changed a lot (it was actually 1947) but I think the integrated circuit in 1958-59 was really the inflection point.
I’ll go with a different event during WWI - von Kluck’s turn South leading to the Battle of the Marne. I think Germany had a very good chance of fulfilling the Schlieffen plan and knocking France out of the war had the series of mistakes and brilliant French/British counterattacks not happened. It’s hard to imagine how the 20th century would have played out if WWI ends in September 2014 with a victorious Germany the primary power on the continent.
Nah, they’d been throwing bricks at each other and cutting each other’s paths in dangerous fashion before it got to guns. And I’m pretty sure they escalated on their own account, not due to orders. Ultimately it’s just an example of brutalization of warfare as a conflict draws longer. It did all happen very fast past the first shots fired though.
Establishing the link between smoking and heart and lung disease. The drive to reduce smoking may be the biggest medical advancement ever, in terms of lives saved.
Something that DIDN’T happen: the soldier whose nationality escapes me who had an opportunity to kill Hitler in WWI, and didn’t.
On the medical front: Alexander Fleming goes on vacation and leaves a petri dish unwashed.
I would go with a related event - the invention of the stored program architecture by von Neumann. Wars come and go, but this was the beginning of the next phase of human evolution.
Regards,
Shodan
Around 1980 there was a convention of newspaper editors (remember newspapers?) and just for fun they had a contest, what is the most startling headline imaginable. The winner was:
Archduke Ferdinand found alive
WWI all a mistake
I think the Japanese bombing at Pearl Harbor would be one of the critical events. Had the Japanese not done that then things might have changed substantially with respect to WWII. Even had the US gotten into the war later down the line it would have made a huge difference. Had we not gotten in at all I think the impact would have been even greater.
You swiped that from Toland or someone. 
It is hard to pick from these 20 times we almost started World War Last, but if I had to chose it would be November 9th, 1979, when a computer exercise tape caused a panic:
Got my vote. I’d offer the attack on Pearl Harbor as an alternate because it led to a huge expansion of American industrial capacity and the development of nuclear weapons and yet unlike the other key players in WW2, the U.S. suffered virtually no damage at all within its territory. Canada got similar benefits but on a far far smaller scale. I suppose Australia did as well. Everyone else got clobbered to varying degrees from minor to total.
Invention of the desktop computer.
I actually think that in the case of Archduke Franz Ferdinand, it really wouldn’t have changed things that much had he not been assassinated. I think that the clash of European powers was inevitable, and something would have set it off. If not that day, then one day for sure. It’s why I, like you, picked Pearl Harbor as I think that this was a real pivot point. I’m not convinced that the US would have joined the war had we not been attacked, and certainly we wouldn’t have been as fervent in pursuing it as we were. That event really crystallized and galvanized the American people, putting the majority firmly behind making the sacrifices necessary to pursue the war to the upmost. Without that, at best I see the US sort of half heartedly joining in sometime further down the road, or the allies being overwhelmed or basically giving up and the US having no real option to join at all. I think that the attack on Pearl Harbor is what lead to the world we have today, with the US in the position it’s in.
Which led to the discovery of penicillin, for anyone who may not have known.
The measles vaccine was another; eradicating that disease alone cut the childhood death rate in half, not so much because measles is so dangerous in itself but because the virus inactivates the immune system to varying degrees for as long as 2 years afterwards.
(I’m surprised that nobody has been snarky and mentioned anything to do with OJ Simpson, or when that soccer player tore off her top and ran around the field in her bra after her team won.)
I was going to go with Archduke Franz Ferdinand as well because none of the subsequent events mentioned by other posters would have happened at all without this one event. But then you could also argue that it was the wrong turn by the Archduke’s driver moments before that which took the car into the path of Gavrilo Princip or instead of that how about the formation of the Black Hand Gang or the means of arming them or some other earlier event which bought them into being?
If we therefore accept that Europe was going to fall into war regardless how about the decisions that Germany had taken in the early stages of WWI? The decision to move into France via Belgium has been described as possibly the biggest single mistake in human history because it bought the British into a war they might have otherwise stayed clear of. Germany might have gained a quick success (or failure) in France, won (or lost) some land and the pressures in Europe would have been eased (perhaps) and the rest of the 20th century would have played out completely differently including WWII, the rise of Communism, the development of Nuclear weapons, American interventionism, the entire space race, the advent of computing, the proliferation of media, and the entirety of western culture after 1945.
One other item for consideration. 4 October 1957. A beach-ball sized hunk of metal with batteries and a transmitter that went ‘beep-beep-beep’ suddenly changed the world and ushered in the space race, the moon mission, and eventual cooperation in space.
Sputnik I.