Joseph Biggs, a onetime lieutenant in the Proud Boys, was sentenced on Thursday to 17 years in prison after his conviction on charges of seditious conspiracy for plotting with a gang of pro-Trump followers to attack the Capitol and disrupt the peaceful transfer of presidential power on Jan. 6, 2021.

The sentence, handed down by Judge Timothy J. Kelly in Federal District Court in Washington, kicked off a series of hearings scheduled for this week and next at which punishment will be meted out against the former chairman of the Proud Boys, Enrique Tarrio, and three other members of the group who were convicted of sedition and other serious crimes at a landmark conspiracy trial this spring.

The Justice Department’s prosecutions of the Proud Boys all but decapitated the group’s national leadership, which was formally disbanded after the Capitol attack, and mostly put an end to its involvement in large-scale — often violent — pro-Trump rallies in cities across the country.

For Mr. Biggs, the sentence effectively ended an unusual career that included a stint as a combat soldier, a job as a roving correspondent for the conspiracy theory website Infowars and a leadership role in the Proud Boys at a moment when the far-right group was thrust from the fringes of national politics and into the center of the 2020 election for their backing of President Donald J. Trump.

In court papers filed this month, prosecutors described Mr. Biggs as “a vocal leader” of the Proud Boys and an “influential proponent of the group’s shift toward political violence,” noting that within days of Mr. Trump’s election loss he had declared that the country could face “civil war.”

He noted that he had reviewed several cases involving terrorism, most concerning situations where defendants “were training to fight American troops or planning an act like blowing up a large building.” And he expressed skepticism that the Proud Boys had engaged in that kind of behavior on Jan. 6.

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