If it wasn’t enough for Donald Trump to stun observers by sacking his national security adviser, he then went and appointed John Robert Bolton to the role.

Ever since his time in the Reagan administration in the 1980s, Mr Bolton has courted controversy and divided opinion.

An unapologetic advocate for the ill-fated invasion of Iraq, he has more recently become a bellicose voice for pre-emptive strikes against both North Korea and Iran.

On April 9, Mr Bolton will become the third person to fill the role of national security adviser in the 14 months of the presidency of Mr Trump, who sensationally sacked incumbent HR McMaster on Friday morning (Australian time).

In 2005, Mr Bolton was appointed US ambassador to the United Nations by George W Bush, raising eyebrows around Washington – not least because Mr Bolton had only two years earlier been accused of distorting US intelligence to exaggerate the unproven claim that Iraq possessed weapons of mass destruction.

Despite previously entertaining the idea of running for president, and even tossing up whether to put his name forward for secretary of state, Mr Bolton’s entry into the White House looks likely to radically shake up Mr Trump’s top foreign policy and national security team.

The 69-year-old Republican has been a vocal proponent of pre-emptive military action against North Korea and Iran. In 2003, he notoriously supported the invasion of Iraq.

In an interview with Fox News following Mr Trump’s announcement on Friday morning (AEDT), Mr Bolton said he would play an “honest broker role” and prioritise “what the President says”.

This did not dispel fears Mr Bolton would be a major and hawkish influence on Mr Trump’s approach to foreign policy, particularly on whether to pull the US out the Iran nuclear agreement and the administration’s approach on North Korea.

Megan Stifel, former National Security Council staffer during the Obama administration, responded to Mr Trump’s new appointment to the White House by tweeting: “Now I’m concerned.”

Richard Haass, the head of the Council on Foreign Relations, tweeted “the obvious question is whether John Bolton has the temperament and the judgment for the job”.

One of Mr Bolton’s most recent works is an op-ed piece in The Wall Street Journal titled, “The legal case for striking North Korea first.”

Mr Bolton spoke of the threat of nuclear warheads delivered by ballistic missiles.

“How long must America wait before it acts to eliminate that threat?” Mr Bolton wrote.

“It is perfectly legitimate for the United States to respond to the current ‘necessity’ posed by North Korea’s nuclear weapons by striking first.

“Given the gaps in US intelligence about North Korea, we should not wait until the very last minute.”

His hawkish views about Iran were candidly expressed in his 2015 op-ed piece for The New York Times titled: “To stop Iran, bomb Iran.”

Mr Bolton said the raft of nuclear sanctions against Iran has not been rigorously enforced or adhered to, making war inevitable.

Who is John Bolton?

Mr Bolton graduated from Yale Law School in 1974 and practised law in Washington DC.

He was first married to Gretchen Bolton in 1972, and divorced in 1983. He later remarried Gretchen Smith Bolton, and together they have one daughter, Jennifer Sarah Bolton, who studied at Yale, like her father.

Mr Bolton was appointed under secretary of state for arms control and international security in May 2001, making him the leading US diplomat dealing with issues involving weapons of mass destruction.

He also served in the administrations of Ronald Reagan, George HW Bush and George W Bush in the Justice and State departments.

Before he became US ambassador to the United Nations, Mr Bolton spoke about the “failure” of the international body.

“The Secretariat building in New York has 38 stories. If it lost 10 stories, it wouldn’t make a bit of difference,” he said.

“The United Nations is one of the most inefficient inter-governmental organisations going.”