The American Civil War The American Civil War was one of the costliest wars in the history of the world. The war that occurred after a sectional conflict arose in the United States of America led to the death of many people. The war occurred after a section of 11 Southern states declared their secession from the American union forming the Confederate States of America. This was after the election of Abraham Lincoln as the president of the United States of America in 1860 (Hardesty, Book 4: Sectionalism, Civil War, and Reconstruction (1837 – 1877), para 2).