President Donald Trump's new National Security Advisor John Bolton has said he wants the U.S. to 'prepare for war.'

Bolton will officially replace the current National Security Advisor H.R. McMaster on April 9, the White House announced late Thursday.

In an interview shortly after his announcement, he said 'The U.S. and the West face a lot of threats.'

Hiring and firing: John Bolton, the former ambassador the United Nations and a security hawk will come into the White House to replace General H.R. McMaster next month, ending months of speculation

Unusual tribute: Trump, who was late to a Greek Independence Day Celebration reception in the East Room because of the shake-up, thanked McMaster for his service

'I think the surest way to avoid conflict is to have a strong military capability,' he told Sky News.

'As the ancient Romans used to say: "Si vis pacem, para bellum." If you want peace, prepare for war.'

Bolton, a former U.S. Ambassador to the United Nations, is a hawkish conservative with a pugnacious streak – and a frequent guest on the Fox News Channel.

He is also a fierce opponent of the Obama-era Iran nuclear deal.

At one time a vacillating potential presidential candidate, the plainspoken yet curmudgeonly Bolton will become Trump's third chief national security aide in his 14-month presidency.

'I am pleased to announce that, effective 4/9/18, @AmbJohnBolton will be my new National Security Advisor,' the president tweeted.

'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.'

Bolton was on Fox News within an hour of Trump's tweet, saying 'I didn't really expect that announcement this afternoon.'

'But it's obviously a great honor, it's always an honor to serve our country.'

Trump had clashed with McMaster repeatedly in recent months, telling confidants that he considered the general a long-winded bore.

His departure comes barely a week after the president dramatically ousted former Secretary of State Rex Tillerson in a tweet.

Trump's national security staff appeared in disarray this week after he placed a congratulatory phone call to Russian President Vladimir Putin – who had won a fourth term in a lopsided election that critics called a corrupt 'sham.'

National security aides, it emerged in an embarrassing leak, had cautioned him in written briefing documents: 'DO NOT CONGRATULATE' – in all capital letters – but he did it anyway, following the example of Barack Obama in 2012.

'I was not involved in any way in the preparations for it,' Bolton said during his Fox interview, talking about the congratulations call.

'The election just took place. And I think it's a matter of courtesy more than anything else.'

Bolton appeared on the Fox News Channel within an hour of the White House announcing he'd been hired

But he was visibly upset that someone in Trump's inner circle would blab to the press that the president had ignored the advice of experts.

'When I read about the leak of the notes and the subject of the conversation, I was outraged by it,' he said. 'It recalled earlier in the administration when somebody was leaking transcripts of the president's conversations with foreign leaders. It's completely unacceptable.'

'I think this is really a terrible reflection on the individual or individuals that did this,' he declared.

Bolton was in the West Wing Thursday afternoon – along with his signature brush-like moustache – and met with Trump.

By dinnertime the White House Press Office was a hive of activity, with Press Secretary Sarah Sanders, her deputy Raj Shah and outgoing communications director Hope Hicks huddling behind closed doors and taking brief interruptions only for trips down the hall to the Oval Office.

Because of the backstage frenzy, Trump was a half-hour late to a planned speech at an East Room event commemorating Greek Independence Day.

The president had told top aides that he wanted to put a new National Security Advisor in place before his planned meeting with North Korean despot Kim Jong-Un, a sit-down that is expected no sooner than late May.

A White House official said Thursday that Trump and McMaster 'mutually agreed' that he would 'resign.'

'The two have been discussing this for some time,' the official added. 'The timeline was expedited as they both felt it was important to have the new team in place, instead of constant speculation.'

McMaster, a three-star Army general, said in a statement that he will retire from the armed forces over the summer, passing up what was thought to be a chance to land a command position with a fourth star.

'Throughout my career it has been my greatest privilege to serve alongside extraordinary servicemembers and dedicated civilians,' he said.

'I am thankful to President Donald J. Trump for the opportunity to serve him and our nation as national security advisor. I am grateful for the friendship and support of the members of the National Security Council who worked together to provide the President with the best options to protect and advance our national interests.'

Pack your bags: A White House official said Thursday that Trump and McMaster 'mutually agreed' that he would 'resign.' 'The two have been discussing this for some time,' the official added.

The president said in his own statement that McMaster 'has served his country with distinction for more than 30 years. He has won many battles and his bravery and toughness are legendary.'

Trump credited McMaster with helping to revitalize U.S. relationships in the Middle East and bring North Korea's dictatorship to the brink of negotiations over its nuclear arsenal.

White House Chief of Staff John Kelly called him 'a true solider-scholar whose impact on the U.S. 'will be felt for years to come.'

Bolton's baptism into the Trump administration won't come without growing pains. He remains a firm believer in the wisdom of President George W. Bush's Iraq war, which Trump has routinely cast as a colossal mistake and a waste of money that could have been spent domestically.

He has repeatedly drawn ire from libertarians like Kentucky Republican Sen. Rand Paul, who rhetorically body-slammed him in a 2016 op-ed when there was talk of Bolton becoming Trump's first secretary of state.

The son of a Baltimore firefighter, Paul wrote, was 'a longtime member of the failed Washington elite that Trump vowed to oppose, hell-bent on repeating virtually every foreign policy mistake the US has made in the last 15 years – particularly those Trump promised to avoid as president.'

'All nuance is lost on the man,' he continued. 'The fact that Russia has had a base in Syria for 50 years doesn't deter Bolton from calling for all out, no holds barred war in Syria. Bolton criticized the current administration for offering only a tepid war. For Bolton, only a hot-blooded war to create democracy across the globe is demanded.'

Asked about strident opposition from Paul and other lawmakers, a White House official responded Thursday: 'Why should we care?'

McMaster was originally hired as a quick-fix replacement for Gen. Michael Flynn, who was fired after mere weeks as National Security Advisor because he hid from Vice President Mike Pence and other officials the nature and extent of his contacts with Russia's U.S. ambassador.