It started because both the Federal Government and the New York centric power barons were both going to lose a hell of a lot of money. Ft. Sumter was a pawn.
They didn't care about the slaves. They cared about the money and the power it gave them.
Jefferson Davis' assault on Fort Sumter was the trigger, just as Japanese attack on Pearl Harbor triggered US entry into WWII.
As a trigger, Fort Sumter demonstrated that secessionists were not about to leave peacefully, but rather in rebellion, insurrection and "domestic violence" against the United States.
DiogenesLamp: "They didn't care about the slaves.
They cared about the money and the power it gave them."
No, they cared first of all about the Constitution and the rebellion being waged against it.