<P> The American Civil War ( also known by other names ) was fought in the United States from 1861 to 1865 . As a result of the long - standing controversy over slavery , war broke out in April 1861 , when Confederate forces attacked Fort Sumter in South Carolina , shortly after U.S. President Abraham Lincoln was inaugurated . The nationalists of the Union proclaimed loyalty to the U.S. Constitution . They faced secessionists of the Confederate States , who advocated for states ' rights to expand slavery . </P>
who was at war in the civil war
the Union the Confederate States