If I ever got sick from something like COVID, I’d just go back to medieval Europe and let it spread like wildfire. I guarantee this modern disease would absolutely wipe out a large majority of Europe and everywhere else it spreads in the world. And little to no effort besides waiting for if I ever get it (which I thankfully never have had and hope I never will).

Let the Cuban missile crisis actually happen?

Stop Hitler’s parents from getting together in the first place.

Lowkey, just travelling back may be enough with the amount of diseases we would carry with us

Shoot Arch Duke Ferdinand. Few people even now about it today, but he was subject to a horribly mangled assassination attempt that many scholars belive would have set off a major war in Europe had he died.

Buy a cheap pistol, go back in time, and shoot Gutenberg.

We’d still be living in medieval times.

Go to back to the big bang. You presence alone will butterfly effect the earth out of existence.

My name is Friedrich Trump (aka Frederick Trump) and I already did that and even had a nice hump.

Give the ancient civilization ruled by the most evil ruler modern weaponry and explain how it works

Bring COVID and HIV to the USA in 1929.

Maybe Zika as well.

Bring knowledge of CFCs to a time when we’re able to make them but not able to detect the ozone problems they cause.

How much power do I have?

If I could divert the asteroid that resulted in the K-T event, that’d drastically change history. It may not have stopped dinosaurs from eventually going extinct, but it’d have given them 33 million more years more to evolve, and would certainly have affected mammalian evolutionary history. Maybe, just maybe, raptors would have gotten smart enough.

Ooh! Take back a lot of ravens. They’re almost smart enough already. Heck, I wonder if taking ravens back even earlier would be enough for them to evolve into something dominant. Problem is, they’re not particularly social, and I think that’s been our greatest advantage.

Or: introduce modern octopus to ancient oceans.

Stopping the K-T event is my favorite, though. It would absolutely have changed how life on Earth has evolved since.

30M years between extinction events is about all you get, though.

I really like this question. So: rather than killing Hitler, what if, instead, you killed Stalin? Was it inevitable that a strongman dictator would have taken over, and ruined the potential of communism? I guess we have evidence that the answer is “yes,” in the form of Mao, but weren’t the Chinese communist party(s) greatly influenced by the Soviet model? What if Russia had, instead, developed a more democratic system of government - was it possible, and couldn’t it have affected how China’s developed? But, maybe it is always inevitable that dictators emerge from internal revolutions like this.

Here’s another scenario: what if you stopped Oswald, and prevented Kennedy from being assassinated? He was popular, and likely to win a second term. What would 4 (~5) more years of Kennedy look like?

Go back in time and do something to prevent Vasily Aleksandrovich Arkhipov from becoming the Executive Officer on the B-59 Soviet nuclear sub in October 1962. He’s the guy who talked the Captain and the Political Officer out of launching the nukes when they thought they were being attacked by the US Navy during the Cuban Missile Crisis. His persuasiveness is generally considered to have avoided WWIII from starting then.