Question: What was the pro-slavery group called that Silas Soule clashed with?

full_context: Document 1: The Immortal Ten: The Immortal Ten were a group of militant Kansan abolitionists and Free-Staters who on July 23, 1859, freed John Doy from a Missouri jail, where he was being held for allegedly abducting slaves. The group included George R. Hay, Silas Soule, Charles Doy, Capt. John E. Stuart [Stewart], S. J. Willis, Thomas Simmons, Joseph Gardner, Jacob Senix, Capt. Joshua A. Pike and Major James B. Abbott.
Document 2: Silas Soule: Silas Stillman Soule (July 26, 1838 – April 23, 1865) was an American abolitionist, Kansas Territory Jayhawker, anti-slavery militant, and a friend of John Brown and Walt Whitman. Later, during the American Civil War, he joined the Colorado volunteers, rising to the rank of Captain in the Union Army.
Document 3: Jayhawker: Jayhawkers is a term that came to prominence just before the American Civil War in Bleeding Kansas, where it was adopted by militant bands affiliated with the free-state cause. These bands, known as "Jayhawkers", were guerrilla fighters who often clashed with pro-slavery groups from Missouri known at the time as "Border Ruffians". After the Civil War, the word "Jayhawker" became synonymous with the people of Kansas. Today a modified version of the term, Jayhawk, is used as a nickname for a native-born Kansan, but more typically for a student, fan, or alumnus of the University of Kansas.
Document 4: Battle of Black Jack: The Battle of Black Jack took place on June 2, 1856, when anti-slavery forces, led by the noted abolitionist John Brown, attacked the encampment of Henry C. Pate near Baldwin City, Kansas. The battle is cited as one incident of "Bleeding Kansas" and a contributing factor leading up to the American Civil War of 1861 to 1865.
Document 5: Mount Oread Civil War posts: Lawrence, Kansas was not well defended in the early part of the Civil War. That ended with William Quantrill's devastating guerrilla raid August 21, 1863. By early 1864 Union soldiers were permanently camped on the top and slopes of Mount Oread, then to Lawrence's southwest. It seems the camp was originally named Camp Ewing, after Brig. Gen. Thomas Ewing.
Document 6: Lawrence massacre: The Lawrence massacre, also known as Quantrill's raid, was an attack during the American Civil War (1861-1865) by the Quantrill's Raiders, a Confederate guerilla group led by William Quantrill, on the Unionist town of Lawrence, Kansas.
Document 7: John Brown Farm State Historic Site: The John Brown Farm State Historic Site includes the home and final resting place of abolitionist John Brown. It is located on John Brown Road in North Elba near Lake Placid, New York, where John Brown moved in 1849 to lead freed slaves in farming. In 1855, he moved to Kansas to support his sons' efforts to keep Kansas as a free-state under the popular sovereignty laws, leaving his wife and several of his children behind. Brown returned to visit his family at Lake Placid several times. In 1859, Brown attempted to start a liberation movement among enslaved African Americans by seizing the arsenal at Harpers Ferry, Virginia (now West Virginia). For this, he was tried for treason against the state of Virginia and was hanged.
Document 8: The Suicide's Soliloquy: The Suicide's Soliloquy is an unsigned poem, thought to be written by Abraham Lincoln, first published on August 25, 1838, in "The Sangamo Journal", a four-page Whig newspaper in Springfield, Illinois. News of the poem first arose shortly after Lincoln's assassination when one of Lincoln's personal friends, Joshua Speed, told William Herndon, Lincoln's biographer, that Lincoln had written a poem about suicide. The poem proved to be impossible to track down until 2004, when Richard Miller, an independent writer discovered the supposed poem. Although the poem seems to follow the same themes and style as Lincoln's other works, there is still controversy over whether or not it was actually written by Lincoln.
Document 9: Marais des Cygnes massacre: The Marais des Cygnes massacre ( , , or ) is considered the last significant act of violence in Bleeding Kansas prior to the outbreak of the American Civil War. On May 19, 1858, approximately 30 men led by Charles Hamilton, a Georgian native and proslavery leader, crossed into the Kansas Territory from Missouri. They arrived at Trading Post, Kansas, in the morning and then headed back to Missouri. Along the way they captured 11 Free-Staters, none of whom were armed and, it is said, none of whom had participated in the ongoing violence. Most of the men knew Hamilton and apparently did not realize he meant them harm. These prisoners were led into a defile, where Hamilton ordered the men to shoot. He even shot and fired the first bullet himself. Five men were killed.
Document 10: William T. Anderson: William T. Anderson (1840 – October 26, 1864)—known as "Bloody Bill" Anderson—was one of the deadliest and most brutal pro-Confederate guerrilla leaders in the American Civil War. Anderson led a band that targeted Union loyalists and Federal soldiers in Missouri and Kansas.
Document 11: Livingston's Hideout: Livingston's Hideout was most likely the only permanent Confederate military camp inside Kansas during the Civil War. It was in the very corner of southeast Kansas, in the very corner of Cherokee County, Kansas. It was about 2 mi north of the border with Indian Territory and it was less than 100 ft west of the border with Missouri. It was 5 mi west of Baxter Springs, where a series of Union military posts existed from 1862 to 1863. Thomas R. Livingston became a leader of a group of Confederate guerrillas in the area, becoming first a captain and then a major. He needed locations to hide himself and his guerrillas from pursuing Union troops and this hideout suited the guerrillas well. The guerrillas sought to spy on Union forces and raiding units he found small enough to defeat.
Document 12: Fort Baxter (Kansas): Fort Baxter, also known as Fort Blair, was a small US Army post located in the southeast corner of Kansas near present-day Baxter Springs. This area was known as the Cherokee Strip. It was one of a few Kansas forts attacked by Confederate forces during the American Civil War. At one point the Confederate government claimed authority over the Neutral Lands. Both Union and Confederate troops operated in the area, as did guerrilla forces and militias prevalent in the Kansas-Missouri border area.
Document 13: John Brown (abolitionist): John Brown (May 9, 1800 – December 2, 1859) was an American abolitionist who believed and advocated that armed insurrection was the only way to overthrow the institution of slavery in the United States. Brown first gained attention when he led small groups of volunteers during the Bleeding Kansas crisis of 1856. Dissatisfied with the pacifism of the organized abolitionist movement, he said, "These men are all talk. What we need is action—action!" During the Kansas campaign, Brown commanded forces at the Battle of Black Jack and the Battle of Osawatomie. He and his supporters killed five supporters of slavery in the Pottawatomie massacre of May 1856 in response to the sacking of Lawrence by pro-slavery forces.
Document 14: Quantrill's Raiders: Quantrill's Raiders were the best-known of the pro-Confederate partisan rangers ("bushwhackers") who fought in the American Civil War. Their leader was William Quantrill and they included Jesse James and his brother Frank.
Document 15: Free-Stater (Kansas): Free-Staters were those settlers in Kansas Territory during the Bleeding Kansas era in the 1850s who opposed the extension of slavery to Kansas. The name derives from the term "free state", that is, a U.S. state without slavery.
ground_truth: "Border Ruffians"