WHO ARE THE MILITIAS?
A subculture of armed right-wing groups with varied motivations has long existed in the US.
After President Donald Trump came to power, many have come out of the shadows, most infamously joining the notorious 2017 Unite the Right rally in Charlottesville, Virginia, and this year protesting against Covid-19 restrictions while heavily armed, and confronting anti-police and Black Lives Matter protesters.
The most prominent - the Three Percenters, Oath Keepers, Proud Boys, Boogaloo Bois, and Patriot Prayer - coalesce around anti-authority, anti-leftist and pro-gun rights ideologies. Some are white supremacists with ties to neo-Nazi movements; some see the police as authoritarian enemies; others say they are preparing for a national revolution or race war.
Some also subscribe to the QAnon movement embracing unfounded theories of a "deep state" threat to Mr Trump and a global child-kidnapping conspiracy led by Democrats.
No one knows how many followers the groups have, but it is easily in the thousands, in all areas of the country connected by social media and encrypted messaging.
WHO WERE THE PEOPLE IN THE MICHIGAN PLOT?
Many of the 13 arrested expressed support for the Boogaloo ideology, and several were members of the recently formed local Wolverine Watchmen armed militia.
Boogaloo is a leaderless, loosely-shaped ideology formed around gun culture and the belief of a looming war or insurrection, fought with the left, with a dictatorial government, or over race.
Some of those arrested had joined rallies this year against Michigan Governor Gretchen Whitmer's coronavirus restrictions, wearing paramilitary gear, carrying multiple weapons, and declaring their rights were being violated.
The Wolverine Watchmen regularly undertook firearms training "to prepare for the 'boogaloo', a term referencing a violent uprising against the government or impending politically-motivated civil war", said a Michigan state court filing.
DO THE MILITIAS THREATEN THE ELECTION?
Potentially, yes. Mr Trump has repeatedly called on his followers to go to polling sites to "protect" the vote. "I'm urging my supporters to go into the polls and watch very carefully, because that's what has to happen," he said during his election debate against Mr Joe Biden.
During the debate he also told the violent armed Proud Boys group to "stand by". One of the group's organisers, Joe Biggs, answered on social media: "Well sir, we're ready."
In states that allow the open carry of firearms, there are few rules to prevent militia groups or armed activists from descending on a voting station to watch or protest, as long as they do not directly menace voters.