President Donald Trump’s embattled chief strategist Stephen K. Bannon left his post at the White House on Friday, in the latest high-profile shake-up for the administration.

Bannon was considered key in helping Trump hone his “America First” message on the 2016 campaign trail — a message that conformed to Bannon’s own “populist nationalism” worldview. Once Trump was elected, Bannon exerted powerful influence at the White House as he worked to turn his and the president’s agenda into national policy.

Bannon also allied with Washington insiders who shared his views, including former Alabama Senator Jeff Sessions, now the attorney general, and Sessions’s top aide, Stephen Miller, who is now Trump’s senior policy adviser.

But Bannon quickly ran into trouble after an April Time magazine cover labeled him the “Great Manipulator” and extolled his influence over the president. The suggestion reportedly annoyed Trump, and within weeks Bannon was dropped from his position on the National Security Council.

“No president, and I think probably President Trump in particular, likes to have it look as though some adviser is pulling all the strings,” Dan Balz, a reporter at The Washington Post, told FRONTLINE in the documentary Bannon’s War. “The president was going to put him in his place and remind people, ‘I’m the president and he works for me.'”

Trump hinted again at firing Bannon outright earlier this week at a press conference where he insisted Bannon was “not a racist,” but refused to guarantee his job security. The following day, The American Prospect published an interview with Bannon in which he contradicted Trump’s threats to unleash “fire and fury” on North Korea, saying there was “no military solution” to Pyongyang’s nuclear threat.

On Friday afternoon, the White House released a statement saying that Bannon and John Kelly, the new chief of staff, had “mutually agreed” that today would be Bannon’s last day. “We are grateful for his service and wish him the best,” it said.

Despite his relatively short tenure, Bannon’s influence on the Trump campaign and the first eight months of the administration have left an indelible mark on national politics. In the below clip from Bannon’s War, FRONTLINE examined how Bannon managed to inject his worldview into Trump’s stump speeches and campaign strategy — and ultimately came to shape domestic policy from a “war room” he set up in the West Wing.

With his proximity to the president, Bannon helped introduce some of the most controversial measures under Trump, including the ban on travelers from seven Muslim-majority countries. He also helped craft executive orders on the environment, Obamacare and the wall along the U.S.-Mexico border.

The chief strategist’s worldview was initially refined and promoted on the site Breitbart.com — which Bannon has dubbed “the platform for the ‘alt-right,'” a rebranded term for white nationalism. His hiring and influence in the White House was lauded by white supremacists, including former Ku Klux Klan leader David Duke, who viewed Bannon as an advocate, saying his hiring was “excellent.”

After the Aug. 12 rally of white supremacists and neo-Nazis in Charlottesville, Va. that left one woman dead and injured 30 others, Bannon said supporters of “ethno-nationalism” were “losers” and “a fringe element.”

Robert Costa, a moderator of Washington Week, told FRONTLINE: “As much as Bannon says himself that he has never embraced these people, Breitbart in some respects has given these people a voice, has normalized elements of the conservative right.”

Sarah Childress, Former Series Senior Editor, FRONTLINE