The suspects are accused of having participated in the "military arm" of the organization led by the minor aristocrat and businessman Prince Heinrich XIII Reuss.
The alleged plot is the most high-profile recent case of far-right violence, which officials say has grown to become the biggest extremist threat in Germany.
The organisation led by Reuss was an eclectic mix of characters and included, among others, a former special forces soldier, a former far-right MP, an astrologer, and a well-known chef.
Reuss, along with other suspected senior members of the group, will face trial in the second of the three cases, in Frankfurt in late May.
The group aimed to install him as head of state after its planned takeover.
The alleged plotters espoused a mix of "conspiracy myths" drawn from the global QAnon movement and the German Reichsbuerger (Citizens of the Reich) scene, according to prosecutors.
The Reichsbuerger movement includes right-wing extremists and gun enthusiasts who reject the legitimacy of the modern German republic.
Its followers generally believe in the continued existence of the pre-World War I German Reich, or empire, under a monarchy, and several groups have declared their own states.
Such Reichsbuerger groups were driven by "hatred of our democracy", Interior Minister Nancy Faeser said in Berlin on Sunday.
"We will continue our tough approach until we have fully exposed and dismantled militant 'Reichsbuerger' structures," she added.
'Treasonous undertaking'
According to investigators, Reuss's group shared a belief that Germany was run by members of a "deep state" and that the country could be liberated with the help of a secret international alliance.
The nine men to stand trial in Stuttgart are accused by prosecutors of preparing a "treasonous undertaking" as part of the Reichsbuerger plot.
As part of the group, they are alleged to have aimed to "forcibly eliminate the existing state order" and replace it with their own institutions.
The members of the military arm were tasked with establishing, supplying and recruiting new members for "territorial defence companies", according to prosecutors.
Among the accused are a special forces soldier, identified only as Andreas M. in line with privacy laws, who is said to have used his access to scout out army barracks.
Others were allegedly responsible for the group's IT systems or were tasked with liaising with the fictitious underground "alliance", which they thought would rally to the plotters' aid when the coup was launched.
The nine include Alexander Q., who is accused by federal prosecutors of acting as the group's propagandist, spreading conspiracy theories via the Telegram messaging app.
Two of the defendants, Markus L. and Ralf S., are accused of weapons offences in addition to the charge of treason.
Markus L. is also accused of attempted murder for allegedly turning an assault rifle on police and injuring two officers during a raid at his address in March 2023.
Police swooped in to arrest most of the group in raids across Germany in December 2022 and the charges were brought at the end of last year.
Three-part trial
Proceedings in Stuttgart are set to continue until early 2025.
In all, 26 people are accused in the huge case against the extremist network, with trials also set to open in Munich and Frankfurt.
Reuss will stand trial in Frankfurt from May 21, alongside another ringleader, an ex-army officer identified as Ruediger v.P., and a former MP for the far-right Alternative for Germany (AfD) party, Birgit Malsack-Winkemann.
The Reichsbuerger group had allegedly organised a "council" to take charge after their planned putsch, with officials warning preparations were at an advanced stage.
The alleged plotters had resources amounting to 500,000 euros ($536,000) and a "massive arsenal of weapons", according to federal prosecutors.
Long dismissed as malcontents and oddballs, believers in Reichsbuerger-type conspiracies have become increasingly radicalised in recent years and are seen as a growing security threat.
Earlier this month, police charged a new suspect in relation to another coup plot.
The plotters, frustrated with pandemic-era restrictions, planned to kidnap the German health minister, according to investigators.
Five other suspected co-conspirators in that plot went on trial in Koblenz last May.
(AFP)
