Why did the northerners fight the Civil War?

The US Civil War was war between pro slavery southern states and the Anti slavery Northern states. The southern states (initially 7 and subsequently 11) announced secession from the Union mainly on the issue of slavery. The slavery states were called Confederation states and the states that had not seceded were called Union States. After the announcement of secession hostilities broke out between two in 1861 and continued till 1865. The war ended with Confederacy defeat.