The battle for Kansas began with the Kansas-Nebraska Act of 1854. This act created the territories of, naturally, Kansas and Nebraska, and used the idea of popular sovereignty to establish whether or not slavery would be permitted in the new territories. Popular sovereignty meant that those who settled in these territories would vote on the issue of slavery, which was becoming more contentious in the U.S.Â
Settlers set their sights on Kansas not because they wanted to live in Kansas (Outside of Superman, who would want that?) but because they could help swing the vote one way or another. Plenty of people from Missouri claimed territory in Kansas just so they could vote on the status of slavery. This might have been described as voter fraud, but it was nothing compared to the actual mini civil war that would follow.
Small acts of violence had occurred earlier between settlers in Kansas, but one of the first major acts was the Sacking of Lawrence of May 21, 1856. Lawrence was a town established by free-staters who wanted Kansas to be an anti-slavery state, and pro-slavery aggressors attacked the town. They burned the Free State Hotel and halted the production of free-state newspapers. Only one person died during this attack, but it demonstrated that the war for Kansas was one that people would resort to violence for.
After the Sacking of Lawrence, the world became introduced to one of the biggest badasses in American history, John "Kill All The Slaveowners" Brown. A radical abolitionist, Brown led the Pottawatomie Massacre, in which his band killed five proslavery Kansas settlers. After this, Brown remained a key anti-slavery figure in Bleeding Kansas and led anti-slavery forces at the Battle of Black Jack and Battle of Osawatomie. Oh yeah, because open warfare was a thing people were willing to do for control of Kansas.