Former President Donald Trump’s fiery Jan. 6 speech, made just before the U.S. Capitol riot began, led to his impeachment by the House of Representatives on a charge of “incitement of insurrection.”

But as the Senate prepares for Trump’s trial to resume Feb. 9, Harvard law professor Alan Dershowitz and other scholars have pointed to a 1969 Supreme Court decision that, in their view, gives First Amendment protection to speakers who urge listeners to use force in certain circumstances. The decision strictly defines the legal concept of “incitement.” It is certain to be invoked as a reason Trump could avoid conviction, assuming the trial touches on the riot, rather than other issues.

The case, titled Brandenburg v. Ohio, struck down a law that was used to prosecute Clarence Brandenburg, a Ku Klux Klan leader. Speaking at a rally in rural Ohio in 1964, Brandenburg said “revengeance” [sic] was needed against government institutions for suppressing the Caucasian race. Interestingly, he said that the “revengeance” would be wrought by “marching on Congress July the Fourth, four hundred thousand strong.”

The high court ruled that state laws making it a crime merely to advocate the use of violence violate the First Amendment. Only when the advocacy is aimed at inciting “imminent lawless action,” and is likely to succeed, may government prohibit it, the court stated unanimously.

Scholars who disagree that the Brandenburg ruling protects Trump argue he unequivocally incited imminent lawless action through comments he made shortly before the rally including, “You’ll never take back our country with weakness. You have to show strength and you have to be strong,” and “If you don’t fight like hell, you’re not going to have a country anymore.” Harvard Law School’s Einer Elhauge asserted in a Washington Post column, “Trump’s conduct clearly meets the legal standard that Brandenburg set.”

As the sides invoke the ruling to support their positions in the coming days, here is a primer on Brandenburg v. Ohio.