As a pro-Trump mob stormed the Capitol last week, Eric Foner, one of the nation’s premier Civil War historians, watched in horror.

“I was watching just like anyone else, with my mouth hanging open,” he said in an interview.

But his mind quickly shifted to history, specifically the little-known history of a little-known provision of the 14th Amendment of the U.S. Constitution.

The 14th Amendment, ratified in 1868, is most famously known for providing citizenship and equal protection under the law to anyone born or naturalized in the United States, including formerly enslaved and free Black people.

But as calls emerged almost immediately for President Trump’s ouster and ban from office via the 25th Amendment or impeachment — neither course is expeditious or easily accomplished — Foner began pondering a different remedy provided by Section Three of the amendment, which says:

No person shall be a Senator or Representative in Congress, or elector of President and Vice-President, or hold any office, civil or military, under the United States, or under any state, who, having previously taken an oath, as a member of Congress, or as an officer of the United States, or as a member of any State legislature, or as an executive or judicial officer of any State, to support the Constitution of the United States, shall have engaged in insurrection or rebellion against the same, or given aid or comfort to the enemies thereof.

“Nobody really had heard about this except people like me who study this era,” said Foner, a Columbia University professor and author of numerous books on the Civil War, Reconstruction and Abraham Lincoln. “And then I had other historians emailing me saying, ‘Wouldn’t Section Three apply here if Trump is guilty?’ ”