That would be my pick of time:
I'd prevent Lincoln from getting elected. This would likely prevent the US Civil War, and all the really bad stuff that happened because of it.
I've yet to figure out the locus for the push for the war, though.
Discussion
Hmmm. I can readily believe you wouldn't get the War Between The States without Lincoln, but I'm not convinced that would have prevented a different civil war, or several in parallel.
By my reading the North had huge internal political stresses thanks to urbanisation, monopoly and immigration; and the South had a smaller helping of that plus a century of pretending the decoupling of class and race wasn't happening.
Yes. I agree. That's why I haven't really been able to find "the one thing" that if you changed or prevented, the mess we are currently in would not have happened.
I would much rather have had the southern states peacefully exit the union than to have had the amount of bloodshed, trauma, and enmity that still exists today, plus all the gubment over-reach that was started, fedgov debt, seeds sown for the mechanisms of federal income taxes, Additional bureaus and assorted mechanisms of government expansion that led to partnerships that are, quite frankly, facistic in nature, between the "robber barons" and their tight grip on the reigns of opinion shaping through media to push for things that should have been left to develop as decentralized, distributed systems built at a much closer to the people level.