John Bolton's appointment as US National Security Adviser should have been impossible, if Donald Trump's tweets were set in stone.

In 2013 — long before he was a presidential candidate — Mr Trump tweeted that "all former Bush administration officials should have zero standing on Syria. Iraq was a waste of blood and treasure."

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Now, as one of those former officials, Mr Bolton is set to become the top US authority on national security.

Not only will he influence US policy in Syria and Iraq, but more immediately and more worryingly, his appointment signals a much tougher US stance against Iran and North Korea.

Even by Bush administration standards he was tough, variously described as a "hawk's hawk" and the most "hawkish of hawks" in Washington.

Mr Bolton is known for his support of using military force against the two rogue states.

As the then-top arms control official, he was a staunch advocate of the 2003 war in Iraq and supported the aggressive use of American power in foreign policy.

NBC News White House correspondent Geoff Bennett says John Bolton is "controversial even in Republican circles."

"He is among the most hawkish of neoconservative hawks," Mr Bennett says.

"He was part of the Bush-Cheney crew that claimed Saddam Hussein had weapons of mass destruction.

"He's also a leading hawkish voice on North Korea."

US Ambassador to the UN John Bolton raises his hand to vote against a UN Security Council resolution condemning Israel's recent attacks in Gaza at the UN headquarters in New York November 11, 2006. ( Reuters: Keith Bedford )

Risks of war with Iran, North Korea now 'significantly greater'

New York Times columnist Nicholas Kristof says John Bolton is not just a hawk. He's a "pterosaur."

"The risks of a war with Iran or North Korea are now significantly greater than they were this morning," he says.

As a regular media commentator Mr Bolton has advocated hardline positions on stopping Pyongyang from developing and using nuclear weapons against the US.

He was once removed from a US delegation taking part in six-party talks on North Korea after he called then-leader Kim Jong-il a "tyrannical dictator" who ran a country where "life is hell".

The North Korean government responded to his exclusion by saying "such a human scum and bloodsucker is not entitled to take part in the talks."

Mr Bolton has also advocated getting rid of the 2015 Iran nuclear deal, a pact that Mr Trump has opposed.

His appointment as Mr Trump's third national security adviser continues a recent trend towards more hawkish advisers likely to support the President's foreign policy views.

And it comes at a crucial time for the Trump administration, as it prepares to make key decisions on both North Korea and Iran.

Mr Bolton is likely to add a harder edge to the upcoming talks between Mr Trump and North Korea's leader Kim Jong-un.

Earlier this year he appeared to oppose such talks happening at all.

"If we pay heed to the lessons of history, talking to North Korea would be fruitless," Mr Bolton tweeted in February.

But he shares Donald Trump's views on the 2015 deal to rein in Iran's nuclear ambitions. In January, Bolton described the deal as a "strategic mistake."

"This deal needs to be abrogated and America must craft a new reality that reflects the actions of the Iranian regime," he wrote.

Mr Bolton also supports Mr Trump's controversial decision to open a US embassy in Jerusalem, despite condemnation from Palestinians and world leaders.

But not all of Mr Bolton's views coincide with those of the President.

Donald Trump has repeatedly said the US war in Iraq was a mistake.

And on Russia, the new National Security Adviser has long voiced hardline views against Moscow, which could sit at odds with a President who openly admires Russian President Vladimir Putin.

Conservative hardliners have praised Mr Bolton for his tough stance on foreign policy but he is also seen as one of the most divisive figures in conservative circles.

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In 2005 he was appointed US Ambassador to the United Nations, despite being one of the UN's fiercest critics.

Back in 1994 he was quoted saying the US was the world's "only real power."

"The Secretariat building in New York has 38 storeys. If it lost 10 storeys, it wouldn't make a bit of difference," he said.

Mr Bolton was forced to resign from the UN job the following year after Democrats refused to support his nomination in the Senate.

He has shown similar disdain for other multilateral institutions, including the International Criminal Court.

A divisive appointment

The US Vice President Mike Pence has described Bolton as a "highly respected American patriot."

"With him as Nat Sec Adv, @POTUS will continue to keep America strong and secure."

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Republican Senator Marco Rubio says Mr Bolton is "an excellent choice who will do a great job."

But critics on the other side of politics have slammed the appointment.

Democratic senator Bernie Sanders describes Mr Bolton as "too extreme to be confirmed as UN ambassador in 2005 and is absolutely the wrong person to be national security adviser now."

Fellow Democrat Betty McCollum tweeted that Mr Bolton was a "discredited architect of the Iraq War" and his "appointment puts our national security at risk."

Ordinary Americans too have derided President Trump's announcement.

Many have referenced his 2013 tweet opposing the appointment of any former Bush administration officials.

"Too bad DT doesn't agree with DT anymore," tweeted one.

"You just hired someone who is likely to start another wasteful war!," tweeted another.

"There really is a tweet for every occasion," said a third.