The civil war came six years early in Kansas. From the mid-1850's until the start of the war, Kansas was roiled by brutality from both pro-slavery guerrillas, sometimes known as "border ruffians" from Missouri, and anti-slavery guerrillas as well, such as John Brown. The state became known as "bleeding Kansas."
No one is quite sure of the first use of the word "jayhawk." Whatever its provenance, in the 1850's, the term "jayhawk" became associated with the struggle against slavery.
After the outbreak of the civil war in 1861, one of the anti-slavery guerrillas, Charles "Doc" Jennison, received a command in the newly-formed Kansas militia. He later became colonel of the Seventh Kansas Cavalry which soon became known as the “Independent Mounted Kansas Jayhawkers.” (An 18 year old William Cody, later known as "Buffalo Bill," was one of its first recruits.)
These "jayhawkers" were involved in several skirmishes and raids in western Missouri in the early days of the war. Jennison had the policy that anyone not supporting the union cause would have their property confiscated--and that's if they were lucky.
Jennison's troops were not always careful to distinguish between those who were pro-slavery and and those who were against it. Put another way, their looting was sometimes indiscriminate. This looting came to be called "jayhawking."
Under pressure from the highest ranks of the Army, Major General David Hunter issued General Orders No. 17 in the Department of Kansas on February 5, 1862, declaring marital law in Missouri.
“... the crime of jayhawking shall be put down with a strong hand and by summary process, and for this purpose the trial of all prisoners charged with armed depredations against property or assaults upon life will be conducted before the military commissions ...”
Not everyone, however, considered "jayhawking" in negative terms. In the fall of 1861, for example, Kansas newspaperman, John Speer, encountered wagons of African-Americans on their way from Missouri to free-state stronghold, Lawrence, Kansas. Speer asked if they were runaway slaves and an elderly woman replied they had been taken by “De blessed Kansas Jayhawkers. Dey Jayhawked us!”
Jennison resigned his commission in May, 1862. The following year, pro-slavery guerrilla, William Quantrill, and his raiders sacked the town of Lawrence. Kansas Governor Thomas Carney called on Jennison to raise a regiment of cavalry to protect the border. This regiment became known as the Kansas 15th.
After the war, Jennison was elected to two terms in the Kansas House of Representatives. In 1871, he was elected to the State Senate.
Image: Charles "Doc" Jennison, leader of the Mounted Kansas Jayhawkers