With the flash of my hand I can probably prevent the civil war.
The Boll Weevil, which feeds primarily on the cotton plant devestated south crops in the early 1900's. Say in 1830 something, maybe even 40's or late 50's, the Weevil is transported to say New Orleans.
From there is spreads and by say 1858 it is ravaging all the primary cotton growing areas, and almost single handedly destroying the economies of Georgia, Alabama, and Mississippi. Because no pesticides exsists then, large scale plantation cropping simply is no longer fesible. If it's not profitable, there certainly is no incentive to go to war and die for slavery like in OTL when slavery was the foundation of peoples livelihoods.
If the Civil War is prevented, maybe civil rights isn't as big of a deal. That one is always hard to predict, would racism and jim crow laws exsist to their extent if hundreds of thousands of ancestors hadn't died in a civil war based around African American slaves? I say it would, but probably not to the huge extent it is today. Their certainley would segregation and anti-black voting laws after a large scale freeing of the slaves. But who knows where they go from there.
You also have to remember that slaves weren't all on plantations. What of the slaves working the cattle trails of Texas? The homes of the rich in New Orleans and Charleston? The Tobbacco crops of Virginia? The Boll Weevil didn't detroy their industires, while a widespread desire to see slave states spread across the west would diminsh, it wouldn't go out entirely and who knows how long until slavery is offically throw out by the states themselves, 1870's? 1880's? I could see some diehards holding out until the 1890's!.
Looking past slavery and civil rights though, suddenly thousands upon thousands of Americans aren't dead. I see the west populating faster, I see a modern American population around 400-500 million instead of 300 million. Industrialization and nationalism probably begin earlier and I could see an earlier Spanish-American War, probably in the 1880's instead of 1898. Does this mean the US to this day would own Cuba and the Phillippines, probably not but maybe. Maybe the US goes to war with another dumb power for no good reason? Maybe the US establishes colonies in the race for Africa? Maybe the US gets involved in world politics earlier and finds itself in WWI from the onset. I don't see US involvment hurrying the defeat of Germany and Austria and I certainley don't see them getting involved in the Russian Civil War more than they did, so more than likely the timeline would revert back to what we have today.
Then again im rambling so what do i know