Unit 5 Review: How Expansion Broke the Union, 1844–1877: AP US History Study Guide

PART II — U.S. HISTORY, 1491 TO THE PRESENT · UNIT 5 REVIEW

How Expansion Broke the Union, 1844–1877

How Expansion Made Civil War More Likely

Territorial expansion after the Mexican-American War reopened the question of slavery’s expansion. Repeated compromises failed because they did not resolve whether Congress, territorial settlers, or slaveholders themselves controlled slavery’s future in the West. Kansas-Nebraska destroyed the old Missouri restriction. Dred Scott narrowed the constitutional room for a congressional limit. The Republican Party’s rise made the party system increasingly sectional. Lincoln’s election convinced Deep South leaders that slavery’s political protection was weakening, and secession turned crisis into war. The war then changed the Constitution. Enslaved people’s flight to Union lines, confiscation policy, emancipation, Black military service, and Union victory made slavery’s destruction part of national policy. Reconstruction amendments abolished slavery, created national citizenship and equal protection, and prohibited race-based voting exclusions. Those constitutional changes survived, but political enforcement weakened as white-supremacist violence, restrictive labor systems, court decisions, economic crisis, and northern retreat rebuilt white control across the South.

The Sequence From Texas Annexation to Reconstruction

KEY DATES

DATE	EVENT
1845	Texas annexation.
1846–1848	Mexican-American War.
1850	Compromise of 1850 and stronger Fugitive Slave Act.
1854	Kansas-Nebraska Act; Republican Party forms.
1857	Dred Scott.
1860–1861	Lincoln elected; secession; Fort Sumter.
1863	Emancipation Proclamation; Gettysburg and Vicksburg.
1865	13th Amendment; war ends.
1868	14th Amendment.
1870	15th Amendment.
1877	Federal Reconstruction enforcement retreats.

Use These Facts to Explain the New Meaning of Citizenship

Wilmot Proviso: territorial expansion disrupted old party coalitions.

Free Soil: opposition to slavery’s expansion was broader than immediate abolition.

Kansas-Nebraska: popular sovereignty could intensify conflict rather than solve it.

Secession declarations: slavery was central to the secession crisis.

Emancipation and Black enlistment: enslaved people and federal policy changed the meaning of the war.

Homestead, Pacific Railway, Morrill, and National Banking Acts: Civil War government expanded federal economic power.

13th–15th Amendments: Reconstruction permanently altered the Constitution.

Sharecropping, racial terror, and court limits: legal freedom did not guarantee equal economic or political power.

Practice: The Crisis of Union

Question 1

Which development most directly turned victory in the Mexican-American War into a sectional political crisis?

A. The acquisition of new territory reopened the question of slavery’s expansion
B. The war abolished the two-party system immediately
C. Mexico required the United States to end slavery
D. California entered the Union before the war began