I'm the late 1840s, what led to the question of whether slavery should expand to the territories?

The suppression of slavery in the American republics in the second half in the nineteenth century, was nothing more than a theoretical act, because the black population did not match their rights with those of the rest of the population until as recently as they are the end of the twentieth century. In the United States, for example, the fight against racial segregation reached its peak in the late 1960's.

Through the proclamation of emancipation (promulgated by the president Abraham Lincoln, declaring the release of all slaves in the year 1863 and entered into effect for the first time at the end of the Civil War in 1865), American abolitionists obtained liberation of the slaves in the states in which slavery continued and the improvement of the conditions of black Americans in general. The abolitionist movement paid for the field for the American civil rights movement.

The vast lands in the west and the need of labor force.