Jayhawkers were abolitionists who fought for the Northern cause. They believed strongly in ending slavery. They originated in Kansas prior to the start of the Civil War.
They were murderers and thieves and very undisciplined with very few principles. They often supplied themselves with stolen horses, and stolen supplies from farmers. Their Confederate counterparts, the pro slavery bushwhackers were just as cruel.
They were located mainly on the Border States, primarily in Kansas. Leading one group was a man named James H. Lane, a former US State Senator. He had a pure hatred for the Confederates, calling them wolves, snakes and devils. Lane and his soldiers were nicknamed “Redlegs" or “redleggers," due to wearing red gaiters.
Another popular leader in the movement was Charles Doc Jennison. He organized the 7th Kansas Cavalry. His men along with other groups regularly invaded Missouri from Kansas with the intent to do nothing more than to free slaves, and kill slave owners.
One of the worst acts committed against the people of the south was the massacre at Osceola, Missouri led by James H. Lane. The town was burned down, and several residents were murdered during the attack.