The conviction of four members of the Proud Boys militia for seditious conspiracy is a warning signal for President Trump, who faces his own scrutiny for the role he played in the sacking of the Capitol on January 6, 2021.

The verdict comes as Special Counsel Jack Smith reportedly sat in on Vice President Pence’s testimony before a grand jury at the District of Columbia, lending the credence of his presence to the looming case against Mr. Trump that Attorney General Garland has tasked him with conducting.

While Mr. Trump was not on trial in the just-concluded four-month marathon of a trial, his name was everywhere in the courtroom. The securing of convictions against the Proud Boys sets the stage for Mr. Smith to hand up an indictment of the former president as the man ultimately responsible for the events of January 6.  

Mr. Smith, who is just as adept in front of war crimes tribunals as petit juries, was on hand to hear Mr. Pence’s first words under oath regarding January 6, where he played the constitutionally mandated role of certifying the election’s outcome. He has written of that period, and the pressure exerted by Mr. Trump that marred it, in his memoir, “So Help Me God.”      

The unanimous vote to convict *Henry “Enrique” Tarrio, the chairman of the Proud Boys and three of his confederates — Ethan Nordean, Joseph Biggs, and Zachary Reh — on the Civil War-era charge is further evidence that prosecutors are succeeding in persuading jurors that January 6 was not spontaneous chaos, but pre-planned mayhem.

The four Proud Boys were convicted not only of seditious conspiracy, but also of three separate conspiracy charges, obstructing the Electoral College vote, and tampering with evidence. Conspiracy charges, of various stripes, can be useful to prosecutors because once a conspiracy is established, its existence is a crime apart from, and in addition to, any further illegal acts committed in its furtherance.   

Seditious conspiracy punishes those who “conspire to overthrow, put down, or to destroy by force the Government of the United States, or to levy war against them,” or to “prevent, hinder, or delay the execution of any law of the United States.” Like other species of conspiracy, it requires “two or more persons” and it carries a maximum sentence of 20 years behind bars. 

The Proud Boys themselves left little doubt about who they saw as their inspiration: President Trump. They sought to minimize their own guilt by maximizing his, portraying themselves as foot soldiers of Mr. Trump. Lawyers for the former president would likely respond that he urged the protesters to be “peaceful” that day, and cannot be fairly held culpable for what ensued.  

The lawyer for one of the convicted Proud Boys told jurors that “their commander-in-chief sold them a lie” and that if the “case of United States v. Donald J. Trump” were ever brought, the former president’s admonition to “fight like hell” would be “exhibit one.”

Prosecutors concurred, even as they sought to send the Proud Boys to prison. At the trial, an assistant United States attorney, Conor Mulroe, argued, “These defendants saw themselves as Donald Trump’s army, fighting to keep their preferred leader in power no matter what the law or the courts had to say about it.”

Mr. Mulroe told jurors that the Proud Boys went into the Capitol like “soldiers into a conquered city,” and they did so under a banner rhetorically unfurled by Mr. Trump on a debate stage the previous September, when he told his supporters to “stand back and stand by.”

*Correction: An earlier version of this article misstated the name of the chairman of the Proud Boys.