How did "Bleeding Kansas" contribute to the onset of the Civil War?

As tensions over slavery heated up in the 1850s, Congress decided to allow the citizens living in the Kansas territory to decide for themselves through the ballot whether or not the state would enter the union as a free or a slave state, a well-meant but ultimately disastrous attempt to defuse a volatile situation. This led both sides of the slavery debate, those who believed in freedom for all people and those who believed in slavery, to flood into the territory to try to influence the vote. Tensions escalated sharply, and violent incidents and clashes followed. Pro-slavers called "border ruffians" came over the border from Missouri, a slave state, and burned, looted, and killed in order to intimidate the abolitionists. John Brown, an abolitionist later famous for the Harper's Ferry raid, led a band of people in violence against the pro-slavers. Violence grew and erupted into a situation akin to a civil war. Though Kansas eventually came into the country as a free state in 1861, the events there helped legitimize the idea of violence as a solution to the slavery problem. The bloody events, and their use as propaganda by both sides also increased polarization and hatred, further hardening the uncompromising positions that led to the Civil War.

What is the significance of Bleeding Kansas?

The significance of "Bleeding Kansas" is that this crisis really pushed the North and South apart and had a great deal to do with causing the Civil War.

"Bleeding Kansas" refers to fighting that happened in Kansas over the issue of whether Kansas would be slave or free.  This conflict was triggered when the Kansas-Nebraska Act of 1854 gave Kansas the right to decide on this issue.  After the act passed, forces for and against slavery poured into Kansas and started fighting each other with atrocities on both sides.

The conflict in Kansas helped to cause more hatred between the North and the South.  This, of course, helped to bring about the Civil War.

Why did Bleeding Kansas lead to the Civil War?

Bleeding Kansas is used to describe the period of violence during the settling of territories in Kansas and Nebraska. In 1854, the Kansas-Nebraska Act was passed in response to complaints that settlers could not lay claim to homesteads because both areas had not yet been organized into territories. However, Southern congressmen were not especially enthusiastic about admitting Nebraska and Kansas into the Union as the areas lay north of the 36 30' parallel (this is the area populated by free states). The Missouri Compromise of 1820 had effectively established this imaginary line to separate free and slave states.

Now, with the Kansas-Nebraska Act, both Kansas and Nebraska would get to decide whether either would be a free or slave state. This effectively destroyed the fragile peace secured by the Missouri Compromise. With passage of the controversial Kansas-Nebraska Act, hostilities erupted in a bid to decide the fate of Kansas as either a slave or free state (Nebraska was further north and its fate as a free-state was never in question). Bloody conflict erupted between thousands of Northerners (who descended on Kansas to try to influence the make-up of the new state legislature) and Missourians, who poured over the border into Kansas to vote for a pro-slavery legislature.

The Northerners set up their Free-state legislature in Topeka, while the Missourians eventually moved their pro-slavery state legislature to Lecompton. There were now two opposing state legislatures in Kansas. The violence continued unabated. Pro-slavery factions tarred, feathered, and killed free-state militiamen. The abolitionist John Brown led an attack on pro-slavery men at Pottawatomie Creek. Five pro-slavery men were dragged from their homes and hacked to death. The Marais des Cynges massacre killed five free-state men. The terrible violence continued until a new governor, John W. Geary arrived to restore order in Sept. 1865.  Bleeding Kansas became one of the most important triggers for the Civil War.