it makes us what we are we can't make it.
 

It's an interesting philosophical question, since your question essentially asks, "how much difference does one person make on the historical scale?" For instance, was Julius Caesar the reason that the Roman Republic transitioned to an autocracy, or was the societal situation such that even if Caesar had just become a farmer or something, someone else would've done more or less the same thing? Same thing with Hitler: was he the source of Germany's rush into World War II, or would the sad state of affairs in post-World War I Germany have created another charismatic fascist who would have done most of the same things?

Seen in the microscopic, we see differences that individuals make and we don't perceive them as being part of a pattern, but zoom out to the macroscopic and you start seeing all kinds of patterns and predictable responses and consequences arise from the actions of many individuals -- these forces don't necessarily seem to control a particular individual, but they do seem to control us in aggregate. (I realize I've basically just described the strucuralist point of view).

In light of this line of thought, I dunno if I'd change much, since I'd worry that the person that society produces instead of, say, Hitler would actually be worse (hard to imagine Nazi Germany being worse though).

I might tweak a couple of things: as a Southerner, I'd warn Lincoln, so that he could have lived to champion his Reconstruction plan, so that the South might not have been as impoverished as the Radical Republicans made it (which might have also made the reactionary racism problem a bit better).

I might have tried to warn German Jews about Kristallnacht, to afford them some chance of not being taken by surprise. Warning the Americans about the Pearl Harbor attack might also have spared thousands of lives by shortening the Pacific war (sort of on a World War II jag here...).

Warning the Bush administration that Iraq did not indeed have WMDs would also have bettered out position in the Middle East right now. And what about saving the life of Archduke Ferdinand? Would that have averted World War I, or would it have happened anyway due to the political state of Europe at the time? Just as important: would we be better or worse off today if the old, European monarchies swept away by the two World Wars were allowed to persist farther into the 20th century?

I'd like to go further into the past, but it gets too difficult to separate what was necessary for our society to even exist from what could have been safely averted.
 

Brownbeard,

Wouldn't those same macro trends also affect the time travelers' world, and thus keep sending back a stream of people to stop successive Hitlers?
 

That'd almost be worse, since it would perpetuate the situation in Germany without ever resolving it. My point is that if leaders like Hitler are the products of a troubled society, then it's really that society's problems that are the ultimate cause of things like World War II. Simply eliminating its leaders doesn't make the underlying problems go away -- in the case of Germany, freedom from the puntitive post-World War I reparations and economic assistance to rebuild were needed to correct the underlying issues; as a result of lessons learned about the grave mistakes in the Treaty of Versailels, both were given, at least to West Germany, after World War II (and Japan was treated similarly, rather than being handled like a dangerous criminal the way Germany was in 1918). If the endless stream of Hitlers were merely killed by their assassins from the future, the problems in Germany may have festered further, possibly even destabilizing central Europe into the latter half of the 20th century, or possibly falling victim to a Soviet-sponsored communist revolution. Ironically, the 21st century could end up looking much darker if World War II (by far the most destructive war in human history) hadn't been fought than if it had.