Trump rally gun suspect called a ‘sovereign citizen.’ What’s that?

Vem Miller, the Nevada man arrested at a checkpoint in southern California allegedly en route to a Donald Trump campaign rally, was associated with a far-right movement called the Sovereign Citizens, according to the local sheriff. 
Miller was arrested after police officers questioned him at a checkpoint and noticed his vehicle sported an irregular license plate. The license plate was “one that is homemade and indicative of a group of individuals that claim to be sovereign citizens,” Riverside County Sheriff Chad Bianco told reporters Sunday.
The Secret Service and other federal agencies said Trump was never in danger. Miller was released on $5,000 bail.
But what is the Sovereign Citizen movement? And why might law enforcement be concerned about someone associated with the group?
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The Sovereign Citizen movement is a disparate collection of thousands, if not tens of thousands, of anti-government radicals who believe they’re not subject to local or national laws or authority. Many sovereign citizens, for example, believe they don’t have to pay taxes, and there have been several high-profile examples of members of the movement arguing their case − unsuccessfully −  in the courts.
As opposed to an organized group or organization such as the Oath Keepers or the Proud Boys, Sovereign Citizens are very much a “movement” said Mark Pitcavage, a senior research fellow at the Anti Defamation League’s Center on Extremism.
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“They’re very anti-authoritarian,” Pitcavage said. “They don’t like anyone telling them to do anything they don’t want to do. They don’t like any sort of authority.” 
The U.S. Attorney’s Office said Sunday that “the U.S. Secret Service assesses that the incident did not impact protective operations and former President Trump was not in any danger.”
But members of the Sovereign Citizen movement have been involved in acts of anti-government violence. In 2010, a father and son team of self-proclaimed Sovereign Citizens shot and killed two police officers during a traffic stop in Arkansas before they were gunned down.   
While not associated with any particular party due to their eschewing of politics, the Sovereign Citizen movement has nevertheless expanded and attracted new members during the Trump era. 
According to a recent report by the ADL’s Center on Extremism, the Sovereign Citizen movement has grown in recent years “by appealing to new audiences, including anti-vaxxers, QAnon adherents, and MAGA supporters.”  
“The movement has also spread extensively in jails and prisons around the country.  Sovereign citizens have increased their international presence as well, spreading to a number of countries around the world,” the ADL briefing said.
The movement, which started growing in popularity in the late 1970s, is inspired and promoted by Sovereign Citizen “gurus” and leaders around the country, many of whom espouse legal and philosophical theories connected to the general anti-government ethos, Pitcavage said.
“You’ll find them anywhere in the country,” he said. “It’s quite a large movement.”