Depends. What war are you talking about? If it's WW1, then it's because everyone was stupid at the time and thought their enemies did it and one thing led to another.
Answer:
The American Civil War (April 12, 1861 – May 9, 1865, also known by other names) was a civil war in the United States fought between northern and Pacific states ("the Union" or "the North") and southern states that voted to secede and form the Confederate States of America ("the Confederacy" or "the South").[e] The central cause of the war was the status of slavery, especially the expansion of slavery into newly acquired land after the Mexican-American War.
