At dawn on June 2, 1856, the abolitionist John Brown led a Free-State militia in an attack on the camp of a pro-slavery militia led by Henry Clay Pate that was encamped along the Santa Fe Trail in southeastern Douglas County, Kansas Territory.

Around 100 men fought an intense three hour battle that ended with Henry Clay Pate, the leader of the pro-slavery militia surrendering to Brown. This action became known as the Battle of Black Jack.
Pate, a 24 year-old Virginia native, and his militia were in the field to "get Old Brown" as a response to the Pottawatomie Massacre on the night of May 24-25, 1856, for which Brown was implicated.
They were using this as an opportunity to put pressure on Free-State partisans in the area, Brown was attempting to stop Pate and his men from their anti-Free-State activities, and to rescue two of his sons who had been captured by the proslavery men.

Brown himself called the action "the first regular battle between Free-State and proslavery forces in Kansas".
Previous "Bleeding Kansas" violence consisted of sackings, massacres, and other events in which a more powerful group quickly overwhelmed smaller unarmed or non-resisting groups and individuals.
The Battle of Black Jack was the first armed action in which two forces of comparable strength and determination fought in Kansas. It was the beginning of civil war combat in Kansas, where a growing number of historians agree that the American Civil War began.

The Battle of Black Jack is where John Brown began his armed war on slavery.
One local historian has called the Battle of Black Jack and Brown's raid on Harpers Ferry the bookends of that war.
There are many direct connections between the battle and the raid.

Brown used the bowie knife he captured from Pate in the battle as the model for the 1,000 pikes that he took to arm freed slaves at Harpers Ferry. J.E.B. Stuart, who was in the military detachment sent to force John Brown to release Pate and his men, was later at Harpers Ferry, and was able to identify the man leading the raid as the man he had met in Kansas in the aftermath of The Battle of Black Jack.

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