Soldiers of the 4th U.S. Colored Infantry
Soldiers of the 4th U.S. Colored Infantry defended Washington, D.C. After January 1863, African-American soldiers made up almost ten percent of the Union Army.
Prints and Photographs Division, Library of Congress
Images of the Era: 1851-1877
Unprecedented growth in the 1850s strained the fragile agreements that had kept the nation united, but had also kept it part slave, part free. The addition of each new state to the Union rattled the delicate political balance carved out by compromises in Congress. In 1854, the Kansas-Nebraska Act allowed residents of each of these new territories, rather than Congress, to decide whether to permit slavery. While intending to keep the nation together, this act inflamed sectional tensions, producing open warfare between pro- and antislavery forces in Kansas, and led directly to the Civil War.