What event from the 1850s foreshadowed the bloodshed that was to come over issue of slavery

Bleeding Kansas--1856 the violence emerging between abolitionists and supporters of slavery broke out in Kansas. 

Bleeding Kansas was regarding the vote in Kansas Territory over whether or not slavery would be allowed in the territory. Slave supporters from Missouri and abolitionists flooded into the territory to sway the vote. Riots broke out between the groups foreshadowing the violence to come over the issue of slavery.