Rewriting History: what one decision would you go back and have someone change?

I like thinking about alternative history. There are people like Harry Turtledove who write extensive alternative histories based on whether the South's main general's war plans got to the Northern armies' general in time for the Battle of Antietam. For me there's something appealing about thinking back through complex events in world history and finding critical moments and critical decisions that might have gone another way. I'm also quite taken with the idea that some historical events end up in hindsight looking like perfect storms, where a number of complex variables make the world we now know, but where any one of those variables would have produced a massively different result.

But I'm less interested in thinking about waving a magic wand to change the weather of some day or to change facts on the ground or morale or something like that. What I'm most interested in are situations where someone's individual decision might have dramatically altered the world. Can you identify one decision that happened in the past that you would have that person making it change? How might that set us up in a different reality?

A small note on housekeeping before I let you go. I know this might be a type of topic that walks the fence between something designed for ~talk and something best suited in ~humanities. I think of this as kind of an experiment to see how best to handle topics that straddle two different tildes.