President Donald Trump on Wednesday named Robert O’Brien, a State Department official who has specialized in hostage issues, as his new national security adviser.

“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,” Trump tweeted. “I have worked long & hard with Robert. He will do a great job!”

O’Brien will replace John Bolton, whom Trump fired last week after a string of disagreements.

O’Brien had been on Trump’s list of finalists, and the president had praised him as recently as Tuesday as “fantastic.”

O’Brien was integrally involved in the release of rapper ASAP Rocky by Swedish authorities last month. The rapper had been detained and charged with assault in connection with a June 30 scuffle between his entourage and a 19-year-old man in Stockholm.

Trump had emerged as a vocal supporter of the rapper in the criminal case.

O'Brien had been present in court in Sweden when the rapper’s trial began and had written to Swedish prosecutors urging them to release him.

O’Brien replaces Bolton, who had been one of the most hawkish voices in Trump’s inner circle on a number of issues, including Taliban negotiations and China trade talks.

Bolton, known as a fierce infighter, had few loyal allies internally. He had clashed with many senior members of the administration at times, including Vice President Mike Pence and Secretary of State Mike Pompeo.