US President Donald Trump has named Army Lieutenant-General Herbert Raymond McMaster as his new national security adviser, choosing a military officer known for speaking his mind and challenging his superiors.
McMaster is a highly regarded tactician and strategic thinker but his selection surprised some observers who wondered how the officer would deal with a White House that has not welcomed criticism.
"He is highly respected by everybody in the military and we're very honoured to have him," Trump told reporters at West Palm Beach on Monday.
"He's a man of tremendous talent and tremendous experience."
In a White House statement issued late on Monday, McMaster said he was honoured to serve in his new role, adding: "The safety of the American people and the security of the American homeland are our top priorities."
The president's choice further elevates the influence of military officers in the new administration.
McMaster joins Defence Secretary Jim Mattis and Homeland Security Secretary John Kelly, both retired generals, in Trump's inner circle of national security advisers.
One subject on which Trump and McMaster could soon differ is Russia.
McMaster shares the consensus view that Russia is a threat and an antagonist to the US, while the man whom McMaster is replacing, retired lieutenant-general Michael Flynn, appeared to view it more as a potential geopolitical partner.
Flynn was fired as national security adviser on February 13 after reports emerged he had misled Vice-President Mike Pence about speaking to Russia's ambassador to the US about US sanctions before Trump's inauguration.
The ouster, coming so early in Trump's administration, was another upset for a White House that has been hit by miscues, including the controversial rollout of a travel ban on people from seven Muslim-majority countries, since the Republican president took office in January.
Senator John McCain, chairman of the Senate armed services committee and a frequent Trump critic, praised McMaster as an "outstanding" choice.
"I give President Trump great credit for this decision," McCain said.
The position of national security adviser does not require Senate confirmation.
Trump also named Keith Kellogg, a retired US Army general who has been the acting national security adviser, as chief of staff to the National Security Council.
John Bolton, a former UN ambassador, would serve the administration in another capacity, Trump said.
Kellogg and Bolton were among those in contention as Trump spent the Presidents Day long weekend considering his options for replacing Flynn.
His first choice, retired vice-admiral Robert Harward, turned down the job last week.
Considered a scholarly officer, McMaster, 54, holds a PhD in military history and has authored a book titled Dereliction of Duty: Lyndon Johnson, Robert McNamara, the Joint Chiefs of Staff, and the Lies that Led to Vietnam.
Trump's pick was praised by one of the president's strongest congressional backers, Republican Senator Tom Cotton, as well as representative Devin Nunes, chairman of the House intelligence committee, who noted McMaster's "history of questioning the status quo".