President Trump on Sept. 18 named the State Department’s hostage envoy Robert C. O’Brien as his fourth national security adviser, replacing John Bolton.

“I am pleased to announce that I will name Robert C. O’Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Advisor. I have worked long & hard with Robert. He will do a great job!” Trump tweeted Wednesday morning.

O’Brien was among a list of five contenders Trump named the day before, a list that had apparently been narrowed from about 15 in the days immediately after Bolton’s ouster.

“Everybody wants it badly, as you can imagine,” he said last week. “A lot of people want the job—it’s a great job. It’s great because it’s a lot of fun to work with Donald Trump. It’s very easy actually to work with me. You know why it’s easy? Because I make all the decisions. They don’t have to work.”

The announcement comes about a week after the president fired former National Security Adviser John Bolton on Sept. 10 because Trump “disagreed strongly with many of [Bolton’s] suggestions, as did others in the Administration,” he wrote on Twitter.

Bolton worked in the administrations of former Presidents Ronald Reagan, George H.W. Bush, and George W. Bush and served as George W. Bush’s lawyer during the 2000 Florida recount.

Bolton replaced former National Security Adviser H.R. McMaster in 2018. McMaster replaced former National Security Adviser Gen. Michael Flynn.

O’Brien joined the administration in May 2018 and served under Secretary of State Mike Pompeo, led the U.S. government’s diplomatic efforts on overseas hostage-related matters. O’Brien also served during the Obama administration as co-chairman of the State Department’s public-private partnership for Justice Reform in Afghanistan.

O’Brien beat out Fred Fleitz, a former CIA analyst who served as Bolton’s chief of staff; Lisa Gordon-Hagerty, the administrator of the National Nuclear Security Administration; Keith Kellogg, national security adviser to Vice President Mike Pence; and Ricky Waddell, a former deputy national security adviser under Trump.

O’Brien comes aboard as the administration faces a number of crises in the Middle East, the possibility of a third nuclear summit with North Korean leader, the prospect of reaching a new arms deal with Russia, all of which will require O’Brien’s involvement.