Sorry... 

I would prefer that other people not know what I am thinking in select cases.In fact, if I could change history, I would make us all telepathic so that we will all benefit from enlightened understanding!
1:Make the Medieval Europeans better.They invented nothing but Feudalism,Spent there time blaming the Jews and the Muslims for things that they had nothing to do with(The Plague)
Japan has been using firearms for nearly as long as Europe has.4:Make it so the Japanese had a gun with a Samurai Sword for a Bayonet.
Okay, here's my serious answer.
If there was only a single thing I could alter, I would go to 1500 AD and tell the European monarchs that they are not going to be remembered fondly for colonization, and they should just stick to peaceful trading and evangelizing with the natives.
...which in turn would make expansion something that would be far too desirable to be halted by some dude talking to the kings.You'll still get native societies devastated. The vast majority of the damage wrought by Europeans was by their microbes, not them.
You'll still get native societies devastated. The vast majority of the damage wrought by Europeans was by their microbes, not them.
Ah, yes, I forgot about that. I don't know why I can't also bring back a whole crate of vaccinations, so long as we're time traveling.
A crate? Nice, you saved one whole neighborhood in Tenochtitlan.Ah, yes, I forgot about that. I don't know why I can't also bring back a whole crate of vaccinations, so long as we're time traveling.
It would eventually mutate into an unstoppable killing force to remind humanity that it shouldn't play God. Or even time-traveler for that matter.Pastwatch had the only viable method I've heard: a genetically engineered virus that was non-lethal and conferred immunity to all the major European diseases.
Killing Hitler (or any other modern dictator) when he was just a simple private in WW1, saving millions of lives.
Yes, it could have.The real question is if WW1 could be prevented, or if the nationalism and and trigger-happiness was too strong...