The events of September 11 shook this nation to its core. They stunned the citizenry, exposed our country’s vulnerabilities, and captured every headline. The events portended our nation’s deepest divide.

Long before the 9/11 of 20 years ago, another episode of violence took place on that day in 1851. That September 11 is long forgotten, despite its being one of the country’s most seminal events. Most Americans called it the Christiana Riot. To Quakers and others who opposed slavery, it was known as the Christiana Resistance.

In the early 1850s, America was already at war with itself, over slavery and its expansion into new territories. Abolitionists, while representing a political minority, were highly organized, and 100,000 members of the population had become fugitives from slavery. In response, in 1850, Congress passed the Fugitive Slave Act, making it a federal crime to assist those seeking freedom. The act also ordered the US Marshals to pursue runaway slaves, monetarily incentivizing slave catchers and creating an industry of hunting down Black Americans, whether free or enslaved.

The Christiana Resistance represented the first challenge to the Fugitive Slave Act, culminating in a violent confrontation between fugitive slaves and enslavers. Two men were central to this moment and embodied its essence: William Parker, who was born a slave on the Roedown Plantation in Davidsonville, Md., and William Gorsuch, the son of a plantation owner in northern Baltimore County, Md.

William Parker began contemplating his escape from slavery when he was 10, after seeing family members sold away. Then, one day in 1839, when he was about 17, Parker was too exhausted to go work in the fields. His owner responded by beating him with an oxen whip. As he wrote years later, in 1866, in The Atlantic Monthly, “we grappled, and handled each other roughly for a time, when [he] called for assistance. He was badly hurt. I let go my hold, bade him goodbye and ran for the woods. As I went to the field, I beckoned my brother, who left work and joined me at a rapid pace.” Now fugitives, the Parker brothers made their way north, to Baltimore and then to Christiana, Pa.

Years later, Parker realized that he couldn’t find safety, even in the North. He would later lament that “slaveholder or kidnappers…a party of white men would break into a house, take a man, no one knew where; and again, a whole family would be carried off. There was no power to protect them, nor prevent it.… I vowed to let no slaveholder take back a fugitive, if I could get my eye on them.”

In the early 1840s, in Christiana, Parker formed the Special Secret Committee, a militia and spy network. As Parker described it, they “formed an organization for mutual protection against slaveholder and kidnappers.… Resolved to prevent any of our brethren being taken back into slavery, at the risk of our own lives…whether the kidnappers were clothed in legal authority or not, I did not care to inquire.” Parker and his band had frequent violent confrontations with slave catchers, freeing those who were kidnapped and killing many of the kidnappers.

Into that world came four escaped men: George and Joshua Hammon, Nelson Ford, and Noah Burley, who fled Edward Gorsuch’s plantation in 1849 after being accused of stealing wheat. Gorsuch was confounded at their show of disrespect, thinking himself the benevolent lord who cared well for his servants. Thinking otherwise, the four men fled to Christiana and came under the protection of William Parker and his wife Eliza, who had become his full partner in the Special Secret Committee.

Edward Gorsuch tried all legal methods to retrieve “his property”—to no avail. Then the Fugitive Slave Act was passed. Emboldened, Gorsuch went to Philadelphia to enlist the help of a man notorious for capturing escaped slaves, Deputy US Marshal Henry H. Kline.

Soon the cohort of Kline, Gorsuch, and two of Gorsuch’s sons had grown to a band of 15. They headed out of Philadelphia to Christiana, armed with guns and federal subpoenas. What this group of slave catchers didn’t realize was that Parker’s Secret Society had spies everywhere. One day, an inebriated Kline talked too much at a tavern and was overheard by Samuel Williams, who followed the men onto the train. Soon the word was out that kidnappers were coming to Parker’s home.

At daybreak on September 11, 1851, Gorsuch and Kline’s little army came bursting into Parker’s home. Kline said he had federal warrant for the runaway slaves. Parker yelled down that he “didn’t give a damn about the United States” and “If you come any closer I will break your neck!” Some of those who were hiding out wanted to turn themselves in, but Parker told them never to give up to the slaveholders. Parker threatened to kill Gorsuch after he said he would retrieve his property or die trying. They began loudly exchanging Bible passages peppered with threats. Caster Hanaway, a white Quaker, rode up and was shown the warrant. He refused to help and told Gorsuch’s gang that they should leave

The kidnapping party, unprepared for resistance, let go a hail of bullets when Eliza Parker blew the ram’s horn. It was too late, though. The Black militia came streaming in, armed with farm implements and guns, and the battle began. Parker shot Gorsuch; one of Gorsuch’s sons was badly wounded; and Kline and the others fled.

In Maryland, 6,000 people gathered to demand retribution. Many went north to join other whites rounding up, beating, and arresting Black men wherever they found them. The US Attorney charged 38 Black men and three white men with treason in the largest conspiracy trial in American history. Leading the defense was the abolitionist Congressman Thaddeus Stevens. The nation was gripped by the headlines emblazoned on every newspaper—“A Black revolt, A prominent white citizen killed.”

Parker fled, made his way north to Frederick Douglass’s home. The two had met once while enslaved and knew each other by reputation. Douglass, who helped Parker flee to Canada, later wrote in his autobiography, “I could not look upon them as murderers. To me they were heroic defenders of the just rights of man against manstealers and murderers. So I fed them, sheltered them in my house…. I shook hands with my friends, received from Parker the revolver that fell from the hand of Gorsuch when he died…a token of gratitude and a memento of the battle for Liberty at Christiana.”

Back in Philadelphia, the trial began. The prosecutors could not believe that Blacks could be this organized, so they tried Caster Hanway first. The defense’s closing argument was: “Leveling war against the United States…. Sir, did you hear it? That three harmless, nonresisting Quakers, and eight and thirty wretched, miserable, penniless Negroes, armed with corn cutters, clubs, and a few muskets, and headed by a miller, in a felt hat, without a coat, without arms, and mounted on a sorrel nag, levied war against the United States. Blessed be God that our union has survived the shock.” The jury took only 15 minutes to acquit. The three whites were released and the 38 Black men spent another three months in jail before being acquitted of all charges.

Outrage rumbled across the country. The State’s Attorney wanted to try the men for murder but couldn’t. Headlines now declared, “Civil War—The First Blow Struck.” People cried for blood and Gorsuch became a martyr. For decades to come, Christiana—not Fort Sumter—was known as the first shot of the Civil War.

Back in Baltimore County, Gorsuch’s youngest son, Thomas, was in boarding school. His best friend, who’d become obsessed with the trial and the killing of Thomas’s father, wrote in his 1860 diary, “My bossom friend…as noble a youth as any living. He had two brothers grown to be men. And an old father who loved and was beloved by them.… Two of his negroes committed a robbery, they were informed upon. They nearly beat the informer to death. They ran away from Maryland, came to this state [Pennsylvania]. The father, the two sons, and the boy my playmate, came to this state under the protection of the fugitive slave law (not only to recover their property, but to arrest the thieves who belonged to them).” His best friend’s name was John Wilkes Booth.

After decades of remembrances, the last of the participants died and the Christiana Resistance was lost to our collective memory. But it remains our nation’s first 9/11, and equally significant to the course of our nation.