WHEN Frazier Glenn Miller shot and killed three peoplein Overland Park, Kan., on Sunday, he did so as a soldier of the white power movement: a groundswell that united Klansmen, neo-Nazis and other fringe elements after the Vietnam War, crested with the bombing of the Oklahoma City federal building in 1995, and remains a diminished but potent threat today.

Mr. Miller, the 73-year-old man charged in the killings, had been outspokenabout his hatred of Jews, blacks, Communists and immigrants, but it would be a mistake to dismiss him as a crazed outlier. The shootings were consistent with his three decades of participation in organized hate groups. His violence was framed by a clear worldview.

You can’t predict whether any one person will commit violence, but it would be hard to think of someone more befitting of law enforcement scrutiny than Mr. Miller (who also goes by the name Frazier Glenn Cross). I’ve been studying the white radical right since 2006. In my review of tens of thousands of pages of once classified federal records, as well as newly available archives of Klan and neo-Nazi publications, Mr. Miller appears as a central figure of the white power movement.

The number of Vietnam veterans in that movement was small — a tiny proportion of those who served — but Vietnam veterans forged the first links between Klansmen and Nazis since World War II. They were central in leading Klan and neo-Nazi groups past the anti-civil rights backlash of the 1960s and toward paramilitary violence. The white power movement they forged had strongholds not only in the South, but also in the Pacific Northwest, Colorado, California and Pennsylvania. Its members carried weapons like those they had used in Vietnam, and used boot-camp rhetoric to frame their pursuit of domestic enemies. They condoned violence against innocent people and, eventually, the state itself.

Before his 1979 discharge for distributing racist literature, Mr. Miller served for 20 years in the Army, including two tours in Vietnam and service as a Green Beret. Later that year he took part (but was not charged) in a deadly shooting of Communist protesters in Greensboro, N.C....

In 2009, the Department of Homeland Security issued a nine-page report detailing the threat of domestic terrorism by the white power movement. This short document outlined no specific threats, but rather a set of historical factors that had predicted white-supremacist activity in the past — like economic pressure, opposition to immigration and gun-control legislation — and a new factor, the election of a black president.

The report singled out one factor that has fueled every surge in Ku Klux Klan membership in American history, from the 1860s to the present: war. The return of veterans from combat appears to correlate more closely with Klan membership than any other historical factor. “Military veterans facing significant challenges reintegrating into their communities could lead to the potential emergence of terrorist groups or lone wolf extremists carrying out violent attacks,” the report warned. The agency was “concerned that right-wing extremists will attempt to recruit and radicalize returning veterans in order to boost their violent capabilities.”...