Northern anti-slave organizations encouraged its members to emigrate to Kansas. The New England Emigrant Aid Society, alone, sent 2,000 settlers to Kansas. Most were well armed and spoiling for a fight. Southerners responded in kind, (although in much smaller numbers) and sent their own armed settlers.
When it was time to vote for the first territorial legislature, Southerners poured in from Missouri and voted early and often. The supporters of slavery triumphed in this election and set up a territorial government in Shawnee Mission. The free soilers (those who opposed slavery in Kansas) were outraged by this fraud and set up their own government in Topeka.
As tension mounted, a group of pro-slavery raiders burned part of the free soil town of Lawrence, thus, provoking a civil war in Kansas. In May 1856, John Brown led a band of followers who hacked to pieces a group of five proslavers, in what became known as the "Pottawatomie Massacre".