The Civil War refers to the war fought in the United States from 1861 to 1865 between the Northern states (the Union) and the Southern states that had separated to form the Confederate States of America. This conflict tore the country apart over fundamental disagreements about slavery and whether states had the right to leave the Union.

The war began when eleven Southern states declared they were leaving the United States, partly because they wanted to preserve slavery and feared the new president, Abraham Lincoln, would restrict or end it. The North fought to hold the nation together and, as the war progressed, to end slavery. Over 600,000 soldiers died in four years of fighting, making it the deadliest war in American history.

The Civil War transformed the nation. When it ended with a Union victory in 1865, slavery was abolished throughout the country, and the idea that states could simply leave the Union was permanently rejected. The war's effects echoed for generations: families were divided, cities were destroyed, and the hard work of rebuilding and reuniting the country, a period called Reconstruction, lay ahead.

More generally, a civil war (lowercase) means any war fought between groups within the same country, rather than between different nations. England, China, Spain, and many other countries have experienced civil wars at different points in their histories.