In reaction to the slave insurrections of the previous era, proslavery, antiabolitionist convictions stiffen in the South (and will persist in the region until the American Civil War).

During the years from 1838 to 1849, the U.S. concludes the second Seminole War (1835-1842) and wages war on Mexico (1846-1847) and the Cayuse tribe of eastern Washington and Oregon (1848-1855).

The process of settling the U.S. border with Canada continues in the Aroostook War (1838-39).

John L. O'Sullivan coins the phrase “Manifest Destiny” in his United States Magazine and Democratic Review (July-August 1845).

Expansionist congressmen quickly adopt the term in their debates over the three territorial questions confronting the United States in 1845 and 1846: ;the annexation of Texas, the joint occupation of the Oregon Territory with England, and the prosecution of war with Mexico.

South Carolina urges all slave-holding states to form a united front against interference by the North, while the newly conquered territory of California requests admission as a free state.

Internal politics turn violent with the Buckshot “War” (1838), the Anti-Renter movement (1839-1846), and Dorr's Rebellion (1842), while the mutinies of the Amistad (1839) and the Creole (1841) exacerbate the national debate over slavery.

So too does the landmark suit brought to the Missouri state courts by the enslaved Dred Scott on the grounds that his residence in a free state and a free territory has made him a free man.

Nine Southern states, defending slavery and the right of all Americans to migrate to the Western territories, seek to extend the Missouri Compromise line west to the Pacific.

The Compromise of 1850 postpones the secession of the South while sowing the seeds of future discord.