AP US History Unit 5 Essentials: Key Terms & Big Ideas

Big Idea 1

Slavery was the central cause of the Civil War

Despite the "Lost Cause" mythology claiming states' rights were the cause, secession documents explicitly state slavery as the reason for leaving the Union. The war was fundamentally about whether slavery would expand or be contained — and ultimately, whether it would survive.

Slavery Civil War Causes Causation

Big Idea 2

The Civil War transformed the nature of federal power

The war dramatically expanded federal authority — income taxes, conscription, a national banking system, and ultimately constitutional amendments redefining citizenship. It settled the question of whether states could secede, but raised new ones about federal power.

Federal Power Constitutional Change Transformation

Big Idea 3

Reconstruction promised freedom but largely failed to deliver it

The 13th, 14th, and 15th Amendments were revolutionary in principle — but Reconstruction's failure left Black Americans subject to sharecropping, Jim Crow, and terror for another century. Understanding why Reconstruction failed is essential for understanding American racial history.

Reconstruction Failure Race

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Manifest Destiny

19th-century belief that America was divinely ordained to expand across the continent; coined by John O'Sullivan in 1845.

Manifest Destiny

Annexation of Texas

1845 admission of Texas as a slave state after nine years of independence from Mexico; directly triggered the Mexican-American War.

Manifest Destiny

Mexican-American War

1846–48 conflict over Texas annexation and the Texas-Mexico border; U.S. victory added vast western territory but reignited slavery debates.

Mexican-American War

Treaty of Guadalupe Hidalgo

1848 treaty ending the Mexican-American War; U.S. gained California, Arizona, New Mexico, Nevada, Utah for $15 million (the Mexican Cession).

Mexican-American War

Wilmot Proviso

1846 proposed law banning slavery in territory taken from Mexico; failed to pass but escalated sectional tensions over slavery's expansion.

Mexican-American War

Compromise of 1850

Henry Clay's deal: California free, popular sovereignty in Utah/NM, no slave trade in DC, harsh new Fugitive Slave Act.

Sectional Crisis

Fugitive Slave Act (1850)

Federal law requiring Northerners to return escaped enslaved people, with harsh penalties; enraged Northerners and radicalized many.

Sectional Crisis

Popular Sovereignty

Doctrine that settlers in a territory should decide whether to permit slavery; advanced by Stephen Douglas, used in Compromise of 1850 and Kansas-Nebraska Act.

Sectional Crisis

Kansas-Nebraska Act

Douglas's 1854 law allowing popular sovereignty in Kansas and Nebraska, repealing the Missouri Compromise; split the Democrats and created the Republican Party.

Sectional Crisis

Bleeding Kansas

1854–59 violent guerrilla war between pro- and anti-slavery settlers in Kansas; included John Brown's Pottawatomie Massacre; foreshadowed Civil War.

Sectional Crisis

Dred Scott v. Sandford

1857 Supreme Court ruling that enslaved people were property, not citizens, and that Congress could not ban slavery in territories; enraged the North.

Sectional Crisis

Uncle Tom's Cabin

Harriet Beecher Stowe's 1852 anti-slavery novel; sold 300,000 copies in its first year and galvanized Northern opposition to slavery.

Sectional Crisis

Republican Party

Anti-slavery-expansion party founded 1854 from Whigs and Free Soilers; opposed slavery's spread (not slavery itself initially); elected Lincoln in 1860.

Sectional Crisis

Lincoln's Election (1860)

Lincoln won the presidency without any Southern electoral votes on a platform opposing slavery's expansion; triggered Southern secession.

Civil War

Fort Sumter

Federal fort in Charleston Harbor attacked by Confederates April 1861; the war's opening battle and the trigger that pushed the Upper South to secede.

Civil War

Battle of Antietam

1862 Maryland battle; bloodiest single day in American history (23,000 casualties); Union victory enabled the Emancipation Proclamation.

Civil War

Battle of Gettysburg

1863 Pennsylvania battle that turned the war's tide; Lee's last major invasion of the North was defeated; Lincoln's 'Gettysburg Address' followed.

Civil War

Emancipation Proclamation

Lincoln's January 1863 executive order freeing enslaved people in Confederate states; made ending slavery a war aim and prevented European intervention.

Civil War

Homestead Act (1862)

Civil War law granting 160 acres of public land to settlers who farmed it for five years; encouraged westward migration.

Civil War

13th Amendment

1865 constitutional amendment abolishing slavery (except as punishment for crime); the first of the Reconstruction Amendments.

Reconstruction

14th Amendment

1868 amendment guaranteeing citizenship and equal protection to all persons born in the U.S.; foundational to civil rights law ever since.

Reconstruction

15th Amendment

1870 amendment prohibiting denial of voting rights based on race; protected Black male suffrage during Reconstruction.

Reconstruction

Freedmen's Bureau

1865–72 federal agency providing food, shelter, education, legal aid, and labor contracts to formerly enslaved people; founded Black colleges.

Reconstruction

Black Codes

Southern laws (1865–66) restricting freedpeople's rights — labor, marriage, movement, 'vagrancy' — effectively re-creating slavery.

Reconstruction

Compromise of 1877

Backroom deal ending the disputed 1876 election; Hayes became president, federal troops withdrew from South, ending Reconstruction.

End of Reconstruction