Why did violence broke out in Kansas Bleeding Kansas?

The passage of the Kansas-Nebraska Act in 1854, is the reason for bleeding Kansas. It delt with slavery in the new territories (in the same manner as the Compromise of 1850).

The act created the territories of Kansas and Nebraska and allowed slavery in both of them. The act also provided that when the people of each territory organized as a state, they could decide by popular vote whether to permit slavery to continue. The decision process was called "popular sovereignty."

The first test of popular sovereignty came in Kansas, where the majority of the population voted against being a slave state. However, proslavery forces refused to accept the decision. The situation quickly turned to violence.

In the end, Kansas joined the Union as free state in 1861.