Learn about the history of the US Civil War with iMindsJNR audio learning series for younger minds. A civil war is a war where the two opposing sides come from the same country. It is a war that occurs within a single nation. The US Civil War waged from 1861 to 1865 and was fought between the northern states (making up the Union) and eleven southern states (the Confederacy). A major cause of the US Civil War was the north's condemnation of slavery.