When did the secession crisis that precipitated the Civil War begin? More broadly, when began the longer era of antebellum sectional conflict that culminated in the secession crisis and then Civil War? Some historians see the beginning of this antebellum era in the 1819–20 North-South political conflict over Missouri’s admission to the Union as a slave state. The Show-Me State was the first situated entirely west of the Mississippi to apply for admission to the Union and was geographically situated so that the traditional borders between free and slave states, the Mason-Dixon Line and the Ohio River, could no longer apply. The Missouri Controversy anticipated much of the sectional tension over the western expansion of the “peculiar institution” that moved again to the front burner of American politics with the annexation of Texas in 1845 and the Mexican War of 1846–48. The flames receded temporarily after the Compromise of 1850 seemed to settle the issue of slavery expansion in the southwestern Mexican Cession of 1848. Yet sectional tensions roared back after the Kansas-Nebraska Act of 1854 rescinded the Missouri Compromise’s 36°30′ latitudinal line separating free from slave territories in the vast national domain that had been acquired by the Louisiana Purchase of 1803. The mid-1850s, despite the temporary political florescence of anti-immigrant and anti–Roman Catholic Know-Nothings, saw the birth and rapid growth in the free states of a Republican Party committed to the geographical containment of slavery. To some extent during the Mexican War but especially after the Kansas-Nebraska Act, the notion of a political and judicial Slave Power and its apparent success in using the Democratic Party to promote the security and spread of the South’s peculiar institution shaped the perceptions of increasing numbers of free state residents.
Sectional controversy over slavery in the territories of the trans-Mississippi West, strongly linked with different interpretations of state versus federal authority under the Constitution, did not directly cause the secession crisis and Civil War. Even so, during the 1840s and 1850s, the territorial question sectionalized American politics and made possible the 1860 victory of a northern sectional president.1 During the 1850s, disputes over enforcement of the new Fugitive Slave Law, likewise connected with different views about the