US President Donald Trump has named Lt Gen HR McMaster as his national security adviser.
He will replace Lt Gen Michael Flynn who was fired after just three weeks and three days in the job.
A lieutenant general with the US Army, HR McMaster served in Iraq and Afghanistan, where he worked on a government anti-corruption drive.
Mr Trump's first choice, retired Vice Admiral Robert Harward, turned down the role, citing "personal reasons".
Who is Lt Gen HR McMaster?
Mr Trump has praised Herbert Raymond McMaster as "a man of tremendous talent and tremendous experience" who is "highly respected by everybody in the military".
Gen McMaster served in Afghanistan and Iraq, and is known as a thoughtful, if straight-talking, military strategist, the BBC's Aleem Maqbool reports from Washington.
He does not appear to have close ties to Moscow, and was recently commissioned to study the ways the US could counter some of Russia's military advances, our correspondent adds.
Gen McMaster is no stranger to questioning authority. In a 2014 interview, he said: "The commanders that I've worked for, they want frank assessments, they want criticism and feedback."
Time magazine named him as one of its 100 most influential people in the world in 2014, saying he "might be the 21st Century Army's pre-eminent warrior-thinker".
He criticised the US military's involvement in the Vietnam War in his book Dereliction of Duty.
He has a PhD in US history from the University of North Carolina.
Gen McMaster has said it is "a privilege... to be able to continue serving our nation" and that he looks forward to joining the national security team.
What will his role as national security adviser involve?
The role involves serving as an independent adviser to the president on issues of national security and foreign policy.
It is one of the most senior roles in the US government. Observers say the role's influence varies from administration to administration, but the adviser is seen as one of the president's key confidantes.
The adviser attends the National Security Council, and may act as a broker between different government departments.
The role is not subject to US Senate confirmation.
Why did his predecessor step down?
Gen Flynn stepped down after misleading Vice-President Mike Pence over his conversations with Russia's ambassador to the US.
Mr Flynn is alleged to have discussed US sanctions with Russia's ambassador in calls before his own appointment.
It is illegal for private citizens to conduct US diplomacy.
However, Mr Trump wrote on Twitter that the intelligence leaks about Mr Flynn's conversations were "the real scandal".
Mr Flynn's short tenure led critics to describe Mr Trump's administration as chaotic.
Who else was considered for the job?
Keith Kellogg, who took over as acting national security adviser after the resignation, will now serve as the National Security Council chief of staff.
The other candidates in the running were Robert Caslen, an Army lieutenant general who is the superintendent of the US Military Academy at West Point, and career diplomat John Bolton, who served as George W Bush's ambassador to the UN from August 2005 to December 2006.
Mr Trump says Mr Bolton will also serve his government "in another capacity".
Mr Trump held interviews with the four men at Mar-a-Lago where he spent the third weekend in a row.
He has called Mar-a-Lago, a private property, the "Southern White House".