Why did the American Civil War start?

The American Civil War (1861-65) arose chiefly over the question on Negro slavery. In the 15th Century the Portuguese found a ready market for Negro slaves, which they captured during their expeditions along the African coasts. As the American continent developed, these slaves were eagerly sought to labour on the cotton and tobacco plantations, in mines, or in general farm work. Between 1680 and 1786 more than 2,000,000 slaves were transported and it was not until 1833 that the United Kingdom Parliament passed an Act that set free slaves in its territories.(Qs and As)