Mr Trump abruptly fired John Bolton, who was a vigorous proponent of using U.S. military force overseas, last week.
U.S. President Donald Trump said Wednesday he's naming hostage negotiator Robert O'Brien, who Trump had monitor the criminal case in Sweden against American rapper A$AP Rocky, as his new national security adviser. "He will do a great job", he said.
And even as Trump's transition was racked by chaos in a variety of areas, O'Brien proclaimed, "We may be witnessing the most impressive presidential transition from a national-security standpoint in history".
O'Brien has been a fan of the recently departed Bolton after the two worked together when Bolton was US ambassador to the United Nations in 2005 during George W. Bush's presidency.
Trump personally dispatched O'Brien to help free U.S. rapper A$AP Rocky from Swedish prison earlier this year.
In terms of his hostage work for the State Department, O'Brien wrote in an article in late 2018 that Iran is the "worst kidnapper of Americans". Just an hour before Trump's tweet, the press office announced that Bolton would join Secretary of State Mike Pompeo and Treasury Secretary Steven Mnuchin in a briefing.
While at the United Nations, he worked with Mr Bolton, who was then the ambassador.
Leader rules out possibility of Iran-US talks on any level
The kingdom, though, has not yet said where the attack was launched from or what kind of weapons were involved. Iran says the attacks were launched from Yemen, but the USA and Saudi Arabia have blamed the attacks on Iran.
"He's a very sharp guy", said Kent Lucken, who was a colleague of O'Brien's on two Romney campaigns.
The position does not require Senate confirmation.
Moments before naming Mr O'Brien as his new advisor, Mr Trump announced he was ordering "substantially" increased sanctions against Iran, which is already buckling under USA economic pressure.
O'Brien is the State Department's top hostage Czar and negotiating official. He was a major in the US Army Reserve.
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In his 2016 book, While America Slept, O'Brien detailed what he saw as some of the failures of US foreign policy in recent years, particularly under the Obama administration.
Although Mr Bolton was seen as the ultimate representative of the neo-con wing in the Republican party, cheering for war in Iraq and pushing for regime change in Iran, Mr O'Brien will bring his own hard edge to foreign policy.
O'Brien has a law degree from the University of California-Berkeley and co-founded a law firm in Los Angeles focused on litigation and worldwide arbitration issues.