The American Civil War (also known by other names) was a war fought in the United States from 1861 to 1865. [c] The Civil War is the most studied and written about episode in U. S. history. Primarily as a result of the long-standing controversy over the enslavement of black people, war broke out in April 1861 when secessionist forces attacked Fort Sumter in South Carolina shortly after Abraham Lincoln had been inaugurated as the President of the United States. The loyalists of the Union in the North proclaimed support for the Constitution. They faced secessionists of the Confederate States in the South, who advocated for states' rights to uphold slavery.