The North and South went to war in 1861 primarily due to the issue of slavery. The disagreement over slavery had been growing for several decades, and by the 1860s, it had reached a boiling point. The Southern states, where slavery was legal and a significant part of the economy, viewed the institution as vital to their way of life. They believed that the federal government should not interfere with their right to own slaves. On the other hand, the Northern states, where slavery had been largely abolished or was on its way to being abolished, saw slavery as a morally reprehensible practice that violated the principles of freedom and equality. They were also concerned about the power and influence of the slave states in the federal government. The election of Abraham Lincoln as president in 1860, who opposed the expansion of slavery, further intensified tensions between the North and the South. In response, several Southern states seceded from the Union and formed the Confederate States of America. The Union, led by President Lincoln, considered secession an illegal act and sought to preserve the Union by force. The resulting conflict became known as the American Civil War.