President Donald Trump is replacing White House National Security Adviser H.R. McMaster with John Bolton, a former U.S. Ambassador to the United Nations famed for his hawkish views, in the latest shakeup of his administration.

“I am pleased to announce that, effective 4/9/18, @AmbJohnBolton will be my new National Security Advisor,” Trump said in a tweet. “I am very thankful for the service of General H.R. McMaster who has done an outstanding job & will always remain my friend. There will be an official contact handover on 4/9.”

Trump is reshaping much of his national security team ahead of planned talks with North Korean leader Kim Jong Un in the coming weeks and as he faces a key decision in May on whether to maintain the Iran nuclear deal. Trump fired Secretary of State Rex Tillerson on March 13, saying the two had disagreed over Iran and other matters of foreign policy.

Bolton brings a far more interventionist view into Trump’s inner circle. He is best known for his ardent support of the 2003 U.S. invasion of Iraq and he has since served as a Fox News contributor and senior fellow at the conservative-leaning American Enterprise Institute. He has also been a frequent behind-the-scenes visitor to Trump in the Oval Office and had campaigned openly to replace McMaster in recent weeks.

Bolton downplayed the significance of his past public statements in an interview with Fox News shortly after the appointment was announced, saying he would defer to the president’s judgment.

“I’ve never been shy about what my views are,” Bolton said. But, he added, that “now is behind me, at least effective April 9, and the important thing is what the president says and what advice I give him.”

Step to the Right

Bolton’s appointment marks another step to the right as Trump swaps out foreign policy moderates. Tillerson, who like McMaster had defended the Iran nuclear deal and counseled diplomacy with North Korea, was fired March 13. Central Intelligence Agency Director Mike Pompeo, who has pushed for a much harder stance against the two countries, is set to replace him.

The move installs Trump’s third national security adviser in 14 months. McMaster joined the administration a year ago after Trump fired his predecessor, Michael Flynn, for lying to the vice president about contacts with Russia. An Army lieutenant general, McMaster has traveled with Trump to several countries and helped craft the president’s national security approach to North Korea, Afghanistan, Iran and other global hotspots.

Some top White House officials knew McMaster was preparing for an eventual exit but wanted him to stay as long as possible, noting Trump is essentially in between secretaries of state, two people familiar with the matter said. They pointed out that McMaster has a strong relationship with South Korean officials, a crucial asset as Trump looks to deal with North Korea.

Jared Kushner and Ivanka Trump supported McMaster and wanted him to have as graceful an exit as possible, but also back the hiring of Bolton. Kushner’s good relationship with Bolton dates to the campaign, and Bolton has been seen in Kushner’s West Wing office frequently in recent months.

Trump and McMaster had clashed during his tenure, which included the dismissal of several aides from the National Security Council.

Public Rebuke

Trump publicly rebuked McMaster in February, saying in a tweet that his national security adviser had neglected to defend his 2016 victory when discussing U.S. claims that Russia meddled in the election.

McMaster said in a statement released by the White House that he would retire from the military this summer.

“Throughout my career it has been my greatest privilege to serve alongside extraordinary servicemembers and dedicated civilians,” McMaster said in the statement. “I am thankful to President Donald J. Trump for the opportunity to serve him and our nation as national security advisor.”

Trump said in an accompanying statement that McMaster “has won many battles and his bravery and toughness are legendary. General McMaster’s leadership of the National Security Council staff has helped my administration accomplish great things.”

The change was welcomed by Republicans. Senator Marco Rubio of Florida applauded Bolton as “an excellent choice who will do a great job.”

Democratic Senator Tim Kaine of Virginia blasted Bolton as “Disqualified by horrible judgment and dangerous for America and the world.”

Pre-emptive Strike

In addition to his hard line on Iran, Bolton has taken a tough stance toward North Korea.

Back in 2003, North Korean state media labeled Bolton “human scum” and “bloodsucker.” In a February Op-Ed in the Wall Street Journal, Bolton laid out the legal case for a pre-emptive strike against North Korea, calling such a move “perfectly legitimate.”

“One way to avoid preemptive military force is to bring the regime down — frankly it’s something that China should help us with,” Bolton said on Fox News March 11. In the same segment, he recalled a joke he’s fond of telling: “How can you tell when the North Koreans are lying? When their lips are moving.”

Trump had initially considered Bolton for secretary of state, and later offered him the job of Tillerson’s deputy — a position he turned down. Bolton had also been considered for the national security adviser job after the firing of Michael Flynn, and though Trump chose McMaster, the president went out of his way to praise Bolton, saying he “had a good number of ideas that I must tell you I agree very much with.”

Bush Alumnus

Bolton served as undersecretary of state for arms control under Bush, and went to the UN as a recess appointment because the administration couldn’t overcome bipartisan opposition in the Senate to his nomination. Bolton left the job after Democrats gained control of the Senate in 2006 and it became clear he’d never win confirmation.

Bolton — whose Twitter profile features seven pictures of himself — has been a frequent visitor to the White House, and while he was open with associates about his desire to replace McMaster, Trump’s own aides consistently downplayed that possibility.

“He is a friend of the president, somebody who he wanted to visit with,” Trump spokeswoman Sarah Huckabee Sanders said after one such meeting in December. “Nothing more than that, nothing more than a check-in and a friendly visit.”

Bolton’s supporters and those who have spent time with him say he is far less confrontational in private. While the decision to name him ambassador to the United Nations was seen as an affront to the institution — he had been a frequent critic of the world body — he generally got along with his fellow ambassadors during his 16 months on the job.

He became known for his willingness to negotiate for as long as it took on the wording of Security Council resolutions, frequently telling reporters “If it’s all night, it’s all right.”