In an alternate timeline, the US Civil War ends very quickly.
Either a different Bull Run leads the Union to march on Richmond, or Virginia stays loyal.
What are the long term effects of a Civil War that is only a few months or less?
Well, first, 600,000 men are not killed, and the South is not laid waste.
Second, the Federal government does not spend billions of $.
Third, slavery will not be abolished in the short term.
The political realignment that began in the 1850s is not completed. The Whig remnants in the South which used the Constitutional Union and Opposition labels will remain. They can't join the Republicans, though on many issues (tariffs, internal improvements) they have the same views, because of the slavery question.
It's possible the Southern ex-Whig crowd organizes formally (which OTL they didn't get around to before the war). They could ally with Republicans in Congress, and persist quite a long time as a regional party.
The U.S. probably intervenes in Mexico by 1862, which affects both France and Mexico.
The Republicans probably push through a law excluding slavery from all territories. This will get appealed to the Supreme Court, but the Republicans will have appointed four new Justices, two of them replacing Southerners, and one to a new seat. I think there would be a majority for overturning
Dred Scott.
Southerners will resent this but decide not to fight it. That will take a lot of wind from Republican sails. OTL, after the war, there was a long period of near parity between the parties - the five very close elections of 1876-1892, and frequent shifts in control of the House.
This was despite the loyalties gained by the Republican Party during the war. If the war is a squib, Republicans don't benefit.
Lincoln could have great difficulty winning re-election.
Getting back to slavery - abolitionists will not be in the driver's seat, but there will stiill be agitation for abolition. It's possible that one or more of the border states will enact gradual emancipation.
An issue that was resolved during the war will continue to fester: the discontent of western Virginians with rule from Richmond. West Virginia may still separate in a few years; but will not include its OTL three easternmost counties in the lower Shenandoah Valley. They were included in WV because they were Union-occupied; the people there were not anti-secession.
I don't think there will ever be another attempt at secession. OTL, it was a huge gamble, taken only because of panic at the election of Lincoln. That won't happen again.
Blacks will remain non-citizens for many years.