Unit 5 Rising Sectional Tensions: Expansion, Compromise, and Collapse (1844–1857)

Manifest Destiny

A widely shared 1840s belief that the United States was meant to expand across North America to the Pacific; more a persuasive cultural/political idea than a single official policy.

John L. O’Sullivan

Journalist most associated with coining/popularizing the phrase “Manifest Destiny” in 1845.

Slave Power (conspiracy fear)

Northern belief that slaveholding elites dominated the federal government and would use expansion to entrench slavery.

Annexation of Texas (1845)

U.S. admission of Texas to the Union, viewed by Mexico as hostile and a major step toward war.

Rio Grande border claim

Texas/U.S. claim that the Rio Grande marked Texas’s southern boundary, a key cause of the U.S.–Mexico border dispute.

Nueces River border claim

Mexico’s claim that the Nueces River (north of the Rio Grande) marked the boundary, creating a contested “in-between” zone.

James K. Polk

U.S. president elected in 1844 who supported expansion and sought California and New Mexico, helping drive the push toward war with Mexico.

Mexican-American War (1846–1848)

War between the U.S. and Mexico that ended in U.S. victory, major territorial gains, and intensified conflict over slavery in new territories.

Treaty of Guadalupe Hidalgo (1848)

Treaty that ended the Mexican-American War; Mexico recognized the Rio Grande border and ceded a large region to the United States in return for payment.

Mexican Cession

Territory ceded by Mexico to the U.S. after the war (including present-day California and much of the Southwest), reopening the slavery-in-the-territories debate.

California Gold Rush (1848–1849)

Rapid influx of migrants to California after gold discovery, accelerating population growth and pushing California quickly toward statehood.

Wilmot Proviso (1846)

Proposed (but never enacted) ban on slavery in territory acquired from Mexico; important for revealing increasingly sectional voting patterns.

Sectionalism

Loyalty to the interests of one region (North, South, or West) over the nation as a whole; intensified as regional systems and politics diverged.

Free labor ideology

Northern worldview emphasizing wage labor, opportunity, and social mobility; many opposed the expansion of slavery because it threatened economic opportunity and concentrated wealth.

Popular sovereignty

Policy proposing that settlers in a U.S. territory should vote to decide whether slavery would be allowed, shifting the decision away from Congress.

Compromise of 1850

Package of laws meant to ease sectional tension after the Mexican Cession and California’s push for statehood; traded concessions between North and South.

Fugitive Slave Act of 1850

Law that strengthened federal enforcement for returning escaped enslaved people and penalized aid to fugitives, nationalizing the slavery conflict and provoking Northern resistance.

Abolition of the slave trade in Washington, D.C.

Compromise of 1850 provision ending the buying/selling of enslaved people in D.C. while leaving slavery itself legal there.

Kansas-Nebraska Act (1854)

Law creating Kansas and Nebraska territories and applying popular sovereignty to slavery there, effectively repealing the Missouri Compromise restriction.

Missouri Compromise 36°30′ restriction

Earlier settlement that limited slavery’s expansion by restricting it north of latitude 36°30′ in the Louisiana Purchase; undermined by Kansas-Nebraska.

Bleeding Kansas

Violent conflict in Kansas as proslavery and antislavery forces rushed settlers, disputed elections, and fought to control slavery’s status under popular sovereignty.

Republican Party (mid-1850s)

Political party that formed as antislavery political energy coalesced, especially in response to Kansas-Nebraska; opposed the expansion of slavery.

Dred Scott v. Sandford (1857)

Supreme Court decision holding that people of African descent were not U.S. citizens (per the Court’s interpretation) and that Congress lacked power to ban slavery in the territories, inflaming sectional tensions.

Roger B. Taney

Chief Justice who authored the Dred Scott decision, which strengthened proslavery constitutional claims and undercut legislative compromise.

Second Party System (breakdown)

Collapse of the older national party coalitions in the 1850s as disputes over slavery and expansion pushed politics into more sectional alignment.