If you could go back in time and change just one event to alter the course of history what would it be?    Please keep it respectful.

With all due respect, I'd stop Cristoforo Colombo's trip of 1492.
I'm absolutely not being polemical or sarcastic; I just wonder what would happen if nobody in Europe had discovered America (yes yes, I know Jarl's son already went to Vinland in the Xth Century, that Phoenicians are said to have been in South America 1000 years before Christ, and that some Spanish probably went there at the end of the Paleolithic. Don't take it too seriously  ).
Perhaps I'd still see Mother Earth as a flat square  
Greetings,
Mauro.

Often I'll look at a place like Philadelphia from one of the bridges or from a plane and wonder what it must have looked like six hundred years ago, before Western man ever set foot there.  It must have been heaven on earth.  Fresh, clean water coming down the Delaware River, whales, dolphin, geese in numbers that blotted out the sun, old growth forests with trees hundreds of feet tall.. paradise.  Yes, let's make a new history where America is never spoiled and the only Western people that ever see it are the members of slinging.org.  I believe that we would never crap it up.

I'd go back, very early, and prevent the extinction of certain large mammals. I'm not sure which, yet, but odds are at least one animal was hunted to extinction that could have been domesticated and seen use, and that that didn't happen in many places has had a lot of effects. It led to power differences that led to militaristic colonialism, it led to delayed technological development which is a significant factor in the huge wealth imbalances in the modern world (see: international access to safe water, where some people completely lack it), as well as likely preventing current technology from being far ahead of what it is. Granted, it was only one of the factors in early development, but it was certainly significant, and undoing it would undo a lot of harm that came from it.

Oh, and as for Mauro's example: This wouldn't stop someone like Columbus from coming to the Americas*. It would stop them from having a ridiculous edge in technology (and, for that matter, germs) and as such would prevent them from being able to do what Columbus, and later people like Cortez and Pizzaro did.

*All that takes is one moron who manages to massively screw up the calculations for the circumference of the Earth, ignore numerous people pointing out that their numbers are wrong, and find a backer who also doesn't realize how bad his calculations are and sends him off with half the supplies necessary to actually reach the known land mass. For them to be like Columbus, they also have to be as morally bankrupt as they are stupid, but given that a huge amount of arrogance is basically known, that isn't likely to be an issue.

Uncle Brian wrote on Feb 8^th, 2012 at 1:48am:

You can't make exact predictions, but we can point to broad historical trends, and make very broad predictions as a result. For instance - say, hypothetically, I were to somehow fill in the Mediterranean sea with land, prior to the emergence of agrarian societies. I can't predict exactly what that will do, but can predict increased early trade between what is now Southern Europe and what is now Africa, and can predict decreased naval development in Europe due to the sudden lack of coastline. Moreover, I could state with some certainty that we would not see the bands of cities on the coast if there was no coast, and instead that cities would be more distributed throughout the entire area.

Plus, tribes living in Amazon forest are living in their Eden - they've done it for centuries and are in perfect harmony with their land.
But somebody needs wood and land and is causing these tribes's extinction right now!
Greetings,
Mauro.

I think the Holocaust made Israel stronger,  today they have a badarse special ops called "Shayetet 13" their motto is "Never Again"  referring to the Holocaust.

I propose Rat Man as the wiser slinger on this forum (well, wiser than me, that is  ).
Greetings,
Mauro.