President Trump has named Robert O’Brien as his fourth national security adviser, after ousting John Bolton from the role last week. O’Brien is author of the 2016 book, “While America Slept: Restoring American Leadership to a World in Crisis,” a collection of essays promoting hawkish U.S. foreign policy. O’Brien is a longtime associate of Secretary of State Mike Pompeo and previously served as the top U.S. hostage negotiator. In July, President Trump sent O’Brien to Sweden to win the release of A$AP Rocky, after the 30-year-old rapper’s arrest for allegedly assaulting a 19-year-old man in Stockholm. He’s previously worked as a national security adviser to the presidential campaigns of Republican candidates Ted Cruz, Scott Walker and Mitt Romney.