On November 7, one day after the 2018 midterm elections, U.S. Attorney General Jeff Sessions submitted his resignation letter to President Donald Trump, at Trump’s request.

“We have operated with integrity and have lawfully and aggressively advanced the policy agenda of this administration,” Sessions wrote in his letter to the president.

Sessions’ exit is the latest turn in a tumultuous relationship between the two men that took off back on the Trump campaign trail.

As FRONTLINE recounted in the October documentary Trump’s Showdown, Sessions — a long-serving Republican lawmaker from Alabama — was the first senator to endorse then-candidate Trump: “He was invaluable to the campaign in helping to get President Trump elected,” J.D. Gordon, a former national security and foreign policy advisor for the Trump campaign, told FRONTLINE.

“Because Sessions endorsed Trump, Trump didn’t have to prove that he was a conservative because Jeff Sessions is sort of the Good Housekeeping Seal of Approval for a lot of conservatives, particularly in the South where Trump was very successful,” Cameron Smith, a former Sessions aide, told FRONTLINE in this excerpt:

Trump would go on to appoint Sessions as his attorney general. But as FRONTLINE recounted in this sequence from Trump’s Showdown, the president’s feelings towards Sessions soured when the new attorney general recused himself from overseeing special counsel Robert Mueller’s probe into Russian election interference.

“I think there’s frustration there, frustration that he appointed somebody to be loyal, and that person abdicated responsibility,” Harvard Law School professor Alan Dershowitz told FRONTLINE:

Though Sessions would help to further Trump’s crackdowns on immigration and gang crime at the Department of Justice, his role as a public and private target of the president’s ire would only grow. In fact, insiders told FRONTLINE in the below scene that Sessions nearly quit after being berated by Trump when Mueller’s appointment was announced:

For more on Sessions’ rocky relationship with the president, and the president’s battles with the Department of Justice, the special counsel, and the FBI, stream Trump’s Showdown in full.

Clarification: This story has been updated to clarify that Sessions and Trump’s relationship began prior to Trump’s presidential campaign.