The American Civil War, which lasted from 1861 to 1865, primarily involved two opposing groups: the Union (the Northern states) and the Confederacy (the Southern states).

Union States:

1. California
2. Connecticut
3. Delaware
4. Illinois
5. Indiana
6. Iowa
7. Kansas
8. Kentucky (border state)
9. Maine
10. Maryland (border state)
11. Massachusetts
12. Michigan
13. Minnesota
14. Missouri (border state)
15. Nevada
16. New Hampshire
17. New Jersey
18. New York
19. Ohio
20. Oregon
21. Pennsylvania
22. Rhode Island
23. Tennessee (initially a Confederate state but had Union support in some areas)
24. Vermont
25. Virginia (West Virginia broke away to join the Union)
26. Wisconsin

Confederate States:

1. Alabama
2. Arkansas
3. Florida
4. Georgia
5. Louisiana
6. Mississippi
7. North Carolina
8. South Carolina
9. Tennessee (also mentioned above)
10. Texas
11. Virginia (as a Confederate state before West Virginia's separation)

Aftermath: The Reconstruction era that followed the Civil War involved significant political and social changes, particularly in the Southern states. During this period, various policies aimed at rebuilding the South and integrating formerly enslaved people into society were implemented, leading to a complex legacy that included both progress and significant resistance.

If you would like more information on specific states, battles, or the effects of the Civil War and Reconstruction, feel free to ask!