A low point has now came in the form of a controversial book in which Bannon, a former White House chief strategist, made what Trump’s lawyers called “disparaging” comments about the president and his family.
Here is a look back at the long and complicated relationship between the two men.
2010: Their first meeting
Aug. 16, 2016: Bannon joins the Trump campaign
"I have known Steve and Kellyanne both for many years,” Trump said in a statement released that morning. “They are extremely capable, highly qualified people who love to win and know how to win.”
Trump brought on Bannon because he respected his instincts and successes, a senior campaign source told ABC News at the time. Trump liked that Bannon was a "high energy person" with "big ideas," the source said.
Oct. 19, 2016: Touting a win
Bannon spoke to the media rarely during the campaign, with one of the few interviews taking place by chance.
"Right now he really, really thinks he's going to win," Bannon said to CNN's Brian Stelter and Brooke Baldwin, who ran into him at the Las Vegas airport before the third presidential debate in October 2016.
"What the media misses is the amount of anger that's out there. Trump didn't create that," Bannon said.
Nov. 13, 2016: A new role for Bannon
Trump created the role of chief strategist and senior counselor to the president for Bannon after their electoral win.
“I am thrilled to have my very successful team continue with me in leading our country,” Trump said in a statement. “Steve and Reince are highly qualified leaders who worked well together on our campaign and led us to a historic victory. Now I will have them both with me in the White House as we work to make America great again.”
January 28, 2017: Bannon given new access
April 5, 2017: Has access curtailed
"He was there to babysit [now-former national security adviser Michael] Flynn, to watch him as he deoperationalized the NSC from [Obama-era national security adviser Susan] Rice," the source said at the time. "Mission done."
April 11, 2017: Trump keeping Bannon at arm’s length
Trump had some choice words about Bannon during an interview with New York Post columnist Michael Goodwin that had some people reading tea leaves for insight into their relationship.
“I like Steve, but you have to remember he was not involved in my campaign until very late,” Trump told Goodwin. "I had already beaten all the senators and all the governors, and I didn't know Steve. I’m my own strategist, and it wasn't like I was going to change strategies because I was facing crooked Hillary.”
“I am my own strategist,” Trump said.
After the violent clashes in Charlottesville, Virginia, the president fielded questions about his response to the protests and also spoke about Bannon, whom many associate with the sometimes perceived racist views touted by Breitbart News, where Bannon worked before (and after) his time in the White House.
Trump told reporters at Trump Tower Bannon was a "good man” and “not a racist.”
"I like Mr. Bannon. He's a friend of mine. But Mr. Bannon came on very late. You know that. I went through 17 senators, governors, and I won all the primaries. Mr. Bannon came on very much later than that," Trump said before adding, "but we'll see what happens with Mr. Bannon."
Aug. 16, 2017: Bannon speaks to a reporter
Trump had grown increasingly frustrated with Bannon in recent weeks, according to one senior White House official, and dissatisfaction from within Trump's inner circle was compounded by Bannon’s interview in American Prospect magazine, which was published Aug. 16. Bannon seemed to undercut the president on North Korea in the interview.
"There's no military solution here, they got us," Bannon told the magazine.
Aug. 18, 2017: Bannon forced out of the White House
Bannon was forced to resign from his position at the White House on Friday Aug. 18, nearly seven months into the administration.
A source close to Trump told ABC News it was ultimately the president's decision to dismiss Bannon. The message was delivered from White House chief of state John Kelly, who was with the president at his golf resort in Bedminster, New Jersey. Bannon was at the White House when he received the call that it was time for him to leave.
"We are grateful for his service and wish him the best," White House press secretary Sarah Sanders said in a statement to ABC News.
Bannon had submitted a letter of resignation to the president earlier in the month with an effective date of Monday, Aug. 14, according to sources close to both Bannon and Trump. But amid the fallout from Trump's controversial response to the violence in Charlottesville, days earlier, Bannon's Aug. 14 resignation date came and went as the president considered Bannon's future, sources said. Meanwhile, several top Trump aides continued to make the case that he needed to go.
Aug. 19, 2017: Trump tweets out kind words
Marking the first time that Trump had tweeted about Bannon directly since he joined the campaign, the president posted two tweets about Bannon’s departure and future role at Breitbart News.
“I want to thank Steve Bannon for his service. He came to the campaign during my run against Crooked Hillary Clinton - it was great! Thanks S,” Trump wrote in the first tweet.
“Steve Bannon will be a tough and smart new voice at @BreitbartNews...maybe even better than ever before. Fake News needs the competition!” he wrote in the second.
Bannon spoke to The Washington Post the same day, saying “no administration in history has been so divided among itself about the direction about where it should go.”
Sept. 11, 2017: Bannon’s big interview
Bannon sat down with CBS's Rose in the biggest interview since his departure from the White House, discussing a wide range of issues.
"I think I'm a street fighter. And by the way, I think that's why Donald Trump and I get along so well. Donald Trump's a fighter. Great counter puncher. Great counter puncher. He's a fighter," Bannon said of their similarities.
In the same interview, Bannon said the firing of FBI Director James Comey was the biggest mistake in "modern political history."
During the daily press briefing, hours before the full interview aired, when asked whether Trump was still talking to Bannon, press secretary Sanders said, "I know they’ve had one conversation but I don’t think anything beyond that since he left."
Sept. 12, 2017: Disputing their calls
Bannon spoke at a lunch in Hong Kong Sept. 12, saying at that point he still spoke with the president every two to three days, the Wall Street Journal reported.
Sanders was asked about that comment during the daily press briefing that day, saying “I don’t think they speak, certainly not that frequently. I’m aware of like two conversations that they’ve had, and nothing beyond that.”
Oct. 16, 2017: Trump describes a rosy relationship
During a news conference in the Rose Garden, when asked whether he supported a plan suggested by Bannon and others not to support Republicans in the midterm elections who don't support the president's agenda, Trump said, "Well, I have a very good relationship, as you know, with Steve Bannon. Steve has been a friend of mine for a long time. I like Steve a lot. Steve is doing what Steve thinks is the right thing.”
“Some of the people that he may be looking at, I’m going to see if we talk him out of that, because, frankly, they’re great people," Trump added.
Oct. 30, 2017: Weighing in on Mueller
Bannon reportedly called Trump and advised him to be more aggressive about slowing down special counsel Robert Mueller's investigation into whether Trump’s campaign colluded with the Russians, according to three sources close to Bannon.
Bannon had been advocating for "a much more aggressive legal approach, short of firing Mueller," a source close to Bannon told ABC News.
ABC News reached out to Bannon at the time, and he declined to comment.
Oct. 31, 2017: Sanders shoots it down
Sanders said at the daily press briefing that the president did not support defunding of the special counsel. She also added that he was happy with his legal team.
“I’m not sure what we would push back against since they’ve just come up with ways and shown more and more that there’s no connection between the Trump campaign and collusion with Russia,” Sanders said.
Nov. 17, 2017: Talking allegiances
When asked whether his relationship with Bannon would influence the president’s stance on Roy Moore, the Republican senate candidate in Alabama who had been accused of assaulting and harassing a number of young women decades earlier, Sanders said it would not.
“The president doesn’t have an allegiance to Steve Bannon. The President has an allegiance to the people of this country and nothing else,” Sanders said at the daily press briefing, adding that she was not “aware of” any conversations Trump had with Bannon about the Moore race.
Jan. 3, 2018: “Fire and Fury”
Excerpts from a book about the Trump administration were published, including some disparaging comments Bannon made about the Trump team and family.
At one point in the book “Fire and Fury: Inside the Trump White House,” Bannon is quoted as suggesting that Donald Trump Jr.’s controversial meeting with a Russian lawyer during the campaign in Trump Tower was potentially “treasonous” and “unpatriotic.”
Trump issued a fiery and lengthy statement in response, slamming Bannon and dismissing his involvement in the campaign.
“Steve Bannon has nothing to do with me or my presidency. When he was fired, he not only lost his job, he lost his mind. Steve was a staffer who worked for me after I had already won the nomination by defeating seventeen candidates, often described as the most talented field ever assembled in the Republican Party,” the statement read.
“Now that he is on his own, Steve is learning that winning isn’t as easy as I make it look. Steve had very little to do with our historic victory, which was delivered by the forgotten men and women of this country. Yet Steve had everything to do with the loss of a Senate seat in Alabama held for more than thirty years by Republicans. Steve doesn’t represent my base—he’s only in it for himself,” the statement continued.
In the daily briefing, Sanders said she believed the last time the pair spoke was in "the first part of December."
Bannon spoke out during a call-in segment of the “Breitbart News Tonight” radio show on Sirius XM, sharing kind words about the president.
“The president of the United States is a great man,” Bannon said. “You know I support him day in and day out.
Trump’s legal team issued cease-and-desist letters to Bannon -- demanding that he stop making disparaging comments about the president and his family -- and the author of the book, Michael Wolff, and his publisher in an attempt to stop publication.
Jan. 4, 2018: Changing tunes
When Trump was asked whether Bannon betrayed him with his comments in the book, Trump pointed to the favorable comments Bannon had made the night before.
“I don’t know, he called me a great man last night so he obviously changed his tune pretty quick,” Trump said.
Sanders was asked about Trump and Bannon’s relationship at the daily briefing, saying, “I'm not aware that they were ever particularly close.”
Jan. 5, 2017: Feud continues
The war of words reached a new level after Trump posted two tweets calling Bannon “Sloppy Steve.”
"I authorized Zero access to White House (actually turned him down many times) for author of phony book! I never spoke to him for book. Full of lies, misrepresentations and sources that don’t exist. Look at this guy’s past and watch what happens to him and Sloppy Steve!" Trump wrote in the first tweet.
In the second, he referenced a family of wealthy Republican donors -- the Mercers – who have pulled their financial support from Bannon, as ABC News reported on Jan. 4.
"The Mercer Family recently dumped the leaker known as Sloppy Steve Bannon. Smart!" the other tweet read.