Christiana, in Lancaster County Pennsylvania, (just north of the border with slave state Maryland) was a station on the Underground railroad, and the home of quite a few free blacks (many of them escaped slaves).
In 1850 the "Fugitive Slave Act" was strengthened, making it more legal to pursue runaway slaves into the free states and leaving Canada as the only really safe refuge.
The Blacks (and many Whites) in Christiana continued to operate their underground railway station and armed themselves and devised a system of alarm calls to call for help in case slave hunters arrived to try to apprehend runaway slaves. William Parker, a free black man was the leader of the Underground Railroad in Christiana.
There were several incidents in Christiana, before the famous Battle. One in which an escaped slave was carried off by slave hunters before the resistance could muster, and was never seen again; one where a slave hunter was confronted by an armed gang of free blacks and beat a retreat back to Maryland.
In 1851, Edward Gorsuch, a slave owner from Maryland, had intelligence that a slave who had run away from him was sheltering in Christiana. He mustered a posse of relatives and friends, and arrived in pursuit.
An agent of the Underground Railroad overheard Gorsuch and his posse making plans. The agent arrived in Christiana actually on the same coach as the posse, and raised the alarm. There was a confrontation outside of William Parker's house, with lot of yelling and threats, and more and more free blacks arriving to defend the fugitive slave. Several shots were fired by both sides.
Then some local Quakers arrived. They brokered a brief parlay. Gorsuch appealed to them to uphold the law and support him in his attempt to reclaim his slave. They refused.
Gorsuch then supposedly said, "I will have my property or go to hell!"
At this point, there was a tussle, and someone among the free blacks who had mustered, shot Gorsuch dead. His son rushed forward to aid his father, firing his revolver, and he was shot down with a shotgun (though he later recovered from his wounds). One other of the posse was wounded by gunshots as were two of the free blacks before both sides scattered.
In the aftermath of the Battle of Christiana, William Parker and two other ringleaders of the Underground Railroad were advised that it might be wise to high tail it for Canada, which they did. They sheltered for a while in the home of the famous black abolitionist Frederick Douglass, and it was he who organized the boat that took them to Canada.
The state of Pennsylvania tried to extradite the three from Canada to stand trial for murder, (or at least some lesser charge - it is uncertain if any of the three were the ones who had fired the fatal shot).
The British Government refused the extradition.
The Southern Press was outraged by the affair.
"Unless the Christiana rioters are hung, the Compromise will be a rope of sand and the Free States will obey it only as it may suit their convenience or pleasure." said the North Carolina Chronicle.
One northern white man, Joseph Miller, who was seeking in Maryland for a free black servant of his who had been kidnapped and taken into slave territory was lynched and a note "In revenge for the death of Gorsuch in Christiana" was pinned to his body.
The Northern Press was divided, with some papers deploring the violence and disregard for the law, while the abolitionist press supported the action.
That Gorsuch "should have been shot down like a dog seems to us the most natural thing in the world," said the 'Pennsylvania Freeman', "and the only wonder is that such catastrophe has not occurred in every case where a fugitive slave has been arrested."
President Fillmore sent US marines to Christiana, where they arrested 36 black men and 5 whites who were accused of treason for taking part in the Christiana affair (the greatest number of people simultaneously accused of treason at one time in the history of the US).
At trial, Judge Grier declared that the actions of these men (if they were involved in the affray, many of the arrested had not been) did not amount to treason, and the charges were dismissed. They could have still been tried by Pennsylvania Courts under lesser charges, but this did not happen, and they all went free.
The Christiana affair helped firm up resistance to the Fugitive Slave Act and anti-slavery sentiment in the North. It fueled resentment in the South. It was one of the factors which built up towards the Civil War.
Meanwhile, in Canada, the three ringleader fugitives were briefly feted as heroes, and then forgotten. I am not sure what happened to them after that.
Main Source:
http://testaae.greenwood.com/doc_print.aspx?fileID=GR7967&chapterID=GR7967-292&path=books/greenwood