Post-Mexican War Divisions: Slavery, Territories, and the Road to Civil War

The Mexican–American War (1846–1848) and the Mexican Cession intensified sectional conflict in the U.S. Key divisive issues:

1. Slavery Expansion – Whether new territories would permit slavery; North opposed, South demanded parity.
2. Wilmot Proviso (1846) – Proposed banning slavery in any territory acquired from Mexico; passed House, failed in Senate.
3. Compromise of 1850 – Admitted California free; organized New Mexico/Utah without slavery restriction; enacted Fugitive Slave Act.
4. Kansas–Nebraska Act (1854) – Repealed Missouri Compromise; allowed popular sovereignty on slavery, sparking "Bleeding Kansas" violence.
5. Dred Scott Decision (1857) – Supreme Court ruled Black Americans could not be citizens; invalidated territorial slavery bans.

These crises eroded national unity, leading directly to the Civil War (1861).