Former U.S. Ambassador to the United Nations (U.N.) John Bolton, President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE’s new national security adviser pick, will enter the White House next month carrying a host of views revered by some and feared by others.

Bolton over the years has advocated for a pre-emptive strike on North Korea, called for the U.S. to pull out of the Iran nuclear deal, pushed to expand the Iraq War into Iran, dismissed accusations of Russian interference in the 2016 U.S. presidential election and accused Cuba of having an active biological weapons program.

“During my career, I have written I don’t know how many articles and op-eds and opinion pieces. I have given — I can't count the number of speeches, I’ve had countless interviews ... They’re all out there on the public record. I’ve never been shy about what my views are,” Bolton said on Fox News hours after news of his White House appointment broke.

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Bolton, who made a name for himself as a controversial official under former President George W. Bush, has served as an informal adviser to Trump, been a frequent Fox News commentator and has worked as a senior fellow at the conservative American Enterprise Institute.

Here are five things to know about Bolton's past and present views as the hawkish new senior aide moves to take over for H.R. McMaster as Trump's third national security adviser on April 9:

Bolton has favored pre-emptive military strikes

Bolton has repeatedly called for a pre-emptive strike on North Korea, arguing as recently as last month that the United States must use force to resolve the nuclear standoff with Pyongyang.

“It is perfectly legitimate for the United States to respond to the current ‘necessity’ posed by North Korea’s nuclear weapons by striking first,” Bolton wrote in a Feb. 28 op-ed for The Wall Street Journal.

“Given the gaps in U.S. intelligence about North Korea, we should not wait until the very last minute. That would risk striking after the North has deliverable nuclear weapons, a much more dangerous situation.”

Bolton has taken just as hard a stance against Iran, writing in a Wall Street Journal op-ed in January that the United States should “end the Islamic Republic before its 40th anniversary."

In numerous television appearances and op-eds, Bolton has called for bombing Iran as the best way to stop Tehran’s nuclear arms research.

He's been a controversial boss

The Senate first blocked Bolton's 2005 appointment to become U.S. ambassador to the U.N. in part over claims that the former State Department official had bullied subordinates.

While undersecretary of State for arms control and international security in 2002, Bolton had his staff create a document arguing that Cuba had an active biological weapons program and even transferred such technology to rogue states.

When the State Department’s top bioweapons analyst refused to approve the claim, Bolton attempted to get him and another top official transferred, the analyst told Congress.

The Washington Post reported at the time that Bolton was furious at the analyst, called him into his office to scold him, and called him a “munchkin."

Another former State Department intelligence analyst told the Senate Foreign Relations Committee that he viewed Bolton as a “serial bully.”

In addition, a government contractor working on a project in Moscow detailed in a letter to the Senate panel aggressive steps Bolton would take to talk with her in 1994, including chasing her down a hotel hall and banging on her suite door.

The Republican-controlled Senate eventually failed to confirm Bolton as ambassador, but Bush in August 2005 appointed him to the position while the Senate was on recess.

He's not a fan of the State Department

Bolton has made no secret of his disdain for government agencies and career officials, and has clashed time and time again with State Department officials.

At the time of Bolton’s nomination proceedings in the Senate, 60 retired diplomats sent a letter to committee lawmakers opposing his nomination.

In 2016, shortly after Trump won the presidency, concerns were again raised over Bolton after his name came up as a potential deputy secretary of State.

Sen. Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.) early on promised to block the nomination, telling The New York Times that “there is something to be said for one of the top diplomats in the country being diplomatic.”

Even Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE, Trump's ultimate pick for secretary of State whom he ousted earlier this month, expressed concerns with having Bolton under him, according to the Times.

Others have defended the hawkish Bush administration official this week, including Sen. Marco Rubio Marco Antonio RubioMurky TikTok deal raises questions about China's role Sunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Florida senators pushing to keep Daylight Savings Time during pandemic MORE (R-Fla.).

"John Bolton is a great addition to the White House. Some of the hyperventilating I am seeing online & on cable news is ridiculous. The one’s who should be freaking out are #Iran Maduro #NorthKorea & terrorists. Bad news for them," Rubio tweeted Friday.

Bolton has been a fixture of Fox News and conservative radio

Bolton became a regular on Fox News and conservative talk radio after he left his ambassador role in December 2006.

Hard-leaning conservatives were delighted with his bluster and take-no-prisoners approach while at the U.N., where he famously said that if the headquarters in New York “lost ten stories, it wouldn’t make a bit of difference.”

Conservative viewers also welcomed Bolton's hard-line views on North Korea, Islam, the Obama administration and conflicts in the Middle East. Bolton, boosted by his airtime as a Fox News contributor, even considered a run for president in 2012 and 2016 and regularly speaks at the annual Conservative Political Action Conference outside Washington.

But Bolton’s views have also drawn praise from other areas, including anti-Islam activists known as the counter-jihad movement. The group has championed the false claim that former President Obama is Muslim and was not born in the United States.

He’s at odds with Pentagon leadership on stated military goals

Bolton will enter his new role with views on Iran and North Korea directly opposite those of Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE.

On North Korea, while Mattis has championed diplomacy in dealing with the isolate nation — arguing in January it “should impose reason on Kim’s reckless rhetoric and dangerous provocations” — Bolton has taken a far different tack.

Bolton argued in September on Fox News that the “only diplomatic option left is to end the regime in North Korea.”

“Anybody who thinks more diplomacy with North Korea, more sanctions ... is just giving North Korea more time to increase its nuclear arsenal,” he said.

And although Mattis has privately pushed for Trump to stay in the Iran nuclear deal, Bolton long called for the U.S. to pull out, calling it “fundamentally flawed.”

In the past several years Bolton has also pushed for regime change in Syria and Libya, to be brought about by military action. Pentagon leadership, in contrast, have steered clear of such involvement.

Unlike his soon-to-be predecessor McMaster, Bolton also does not have a prior relationship with Mattis, a retired four-star general.

“I’m not sure I’ve ever met Gen. Mattis,” Bolton said during his Fox News interview.

Additionally, others have noted that Bolton’s views also seem to contradict Trump’s campaign rhetoric of staying out of other countries.

The president on March 4 called the 2003 invasion into Iraq “the single worst decision ever made.”

“Frankly, what I’ve said in private now is behind me," Bolton said on Fox News after his appointment this week. "The important thing is what the president says and what advice I give him.”