Jesse Woodson James (September 5, 1847April 3, 1882) was an American outlaw, bank and train robber, guerrilla, and leader of the James–Younger Gang.
 Raised in the "Little Dixie" area of western Missouri, James and his family maintained strong Southern sympathies.
 He and his brother Frank James joined pro-Confederate guerrillas known as "bushwhackers" operating in Missouri and Kansas during the American Civil War.
 As followers of William Quantrill and "Bloody Bill" Anderson, they were accused of participating in atrocities against Union soldiers and civilian abolitionists, including the Centralia Massacre in 1864.
John Wesley Fribley was born on a farm near Assumption, Illinois, in 1906, and died in Shelbyville, Illinois, in 2002.
 He represented the 40th Senate District in the Illinois State Senate from 1934 through 1952.
 He had significant legislative achievements in transportation, oil and gas regulation, and mine safety.
 He helped investigate the Centralia mine disaster and was co-sponsor of the Illinois Mine Safety Law.
 He sponsored the Farm-to-Market Road Law, which constructed township roads and benefited rural towns in Central Illinois.
 He worked to improve the relationship between Chicago and downstate Illinois.
 Fribley was a downstate floor leader for former Governor Adlai E. Stevenson.
William George Ramsay ("Bill" or "Rams"; born January 12, 1929, in Centralia, Washington) is an American jazz saxophonist and band leader based in Seattle.
 In 1997, he was inducted into the Seattle Jazz Hall of Fame, the top of eight Golden Ear Award categories presented annually since 1990 by the Earshot Jazz Society of Seattle.
 Ramsay performs on all the primary saxophones – soprano, alto, tenor, and baritone – as well as clarinet (his boyhood instrument), and bass clarinet.
Nathan Wesley Everest (December 29, 1890 in Newberg, Oregon — November 11, 1919 in Centralia, Washington) was an American member of the Industrial Workers of the World (IWW) and a World War I era veteran.
 He was lynched during the Centralia Massacre after killing Dale Hubbard in what the union called self-defense, though the American Legion called it murder.
The Centralia Massacre, also known as the Armistice Day Riot, was a violent and bloody incident that occurred in Centralia, Washington, on November 11, 1919, during a parade celebrating the first anniversary of Armistice Day.
 This conflict between the American Legion and workers who were members of the Industrial Workers of the World (IWW or "Wobblies") resulted in six deaths, additional wounded, multiple prison terms, and an ongoing and especially bitter dispute over the motivations and events that precipitated the massacre.
 It was the culmination of years of bad blood between members of the local Legion and members of the IWW.
 Both Centralia and the neighboring town of Chehalis had a large number of World War I veterans, with robust chapters of the Legion, as well as a large number of IWW members, some also war veterans.
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.
Fred H. Moore was a socialist lawyer and the defense attorney of the controversial Sacco and Vanzetti case.
 He had collaborated in many labor and Industrial Workers of the World trials.
 He played a minor role in several celebrated I.W.W. trials, including the Los Angeles Times bombing case in 1911 and the trial of Ettor-Giovannitti case, which arose from the 1912 Lawrence, Massachusetts, textile strike.
 Following acquittal in the Ettor-Giovannitti case, Moore spent the next several years roaming the country defending I.W.W. organizers.
 He was involved in the Centralia Massacre trial and the mass prosecution, on charges of sedition, of the I.W.W. in Chicago in 1918.
 Errors in a later trial, however, led Big Bill Haywood to demand Moore's resignation as I.W.W. attorney in 1920.
 Moore's career was revived by his hiring to head the defense team for Sacco and Vanzetti in the summer of 1920.
Elmer Smith (1888–1932) was a 20th-Century American lawyer and Industrial Workers of the World union defender, who played a key role in the 1919 Centralia massacre (Washington).
Warren O. "Wedge" Grimm (March 9, 1888 – November 11, 1919) was an All-American at the University of Washington and an officer in the United States Army, he served with distinction as part of the American Expeditionary Force Siberia stationed in Russia in 1918–19.
 He was killed on November 11, 1919 during the Centralia Massacre in Washington State.
The Centralia Massacre was an incident during the American Civil War in which twenty-four unarmed Union soldiers were captured and executed at Centralia, Missouri on September 27, 1864 by the pro-Confederate guerrilla leader William T. Anderson.
 Future outlaw Jesse James was among the guerrillas.

Question: What was William T. Anderson the leader of in the Centralia Massacre?
Answer: pro-Confederate guerrilla leaders