In 1861, the United States faced its greatest crisis to that time. The northern and southern states had become less and less alike--socially, economically, politically. The North had become increasingly industrial and commercial while the South had remained largely agricultural. More important than these differences, however, was African-American slavery. The "peculiar institution," more than any other single thing, separated the South from the North. Northerners generally wanted to limit the spread of slavery; some wanted to abolish it altogether. Southerners generally wanted to maintain and even expand the institution. Thus, slavery became the focal point of a political crisis.
Following the 1860 election to the presidency of Republican Abraham Lincoln, 11 southern states eventually seceded from the Federal Union in 1861. They sought to establish an independent Confederacy of states in which slavery would be protected. Northern Unionists, on the other hand, insisted that secession was not only unconstitutional but unthinkable as well. They were willing to use military force to keep the South in the Union. Even Southerners who owned no slaves opposed threatened Federal coercion. The result was a costly and bloody civil war. Almost as many Americans were killed in the Civil War as in all the nation's other wars combined.
After four years of fighting, the Union was restored through the force of arms. The problems of reconstructing the Union were just as difficult as fighting the war had been. Because most of the war was fought in the South, the region was devastated physically and economically. Helping freedmen (ex-slaves) and creating state governments loyal to the Union also presented difficult problems that would take years to resolve.