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 three weeks and three days in the job.
A lieutenant general with the US Army, General 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".
President Trump called General McMaster "a man of tremendous talent and tremendous experience", adding: "He is highly respected by everybody in the military."
Gen Flynn stepped down after misleading Vice-President Mike Pence over his conversations with Russia's ambassador to the US.
Keith Kellogg, who took over as acting national security adviser after the resignation, will now serve as the National Security Council chief of staff.
Gen McMaster is known for expressing his views - 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".
Thanking the US president for the new position, he said: "I would just like to say what a privilege it is to be able to continue serving our nation and I'm grateful to you for that opportunity.
"I look forward to joining the National Security team and doing everything I can to advance and protect the interest of the American people."