US President Donald Trump on Monday named Army Lt. Gen. Herbert Raymond “H. R.” McMaster as his new national security adviser, replacing the ousted Michael Flynn, reported The Washington Post.

Flynn was asked to resign last week amid allegations that he discussed US sanctions with a Russian official before Trump took office and then misrepresented the content of that conversation to Vice President Pence and other administration officials.

McMaster, 54, earned a doctorate in history from the University of North Carolina and turned his thesis into a widely acclaimed book on Vietnam, “Dereliction of Duty.” Relying on declassified documents, McMaster argued that the generals in Vietnam had caved in to political pressure and supported a war strategy they knew could not prevail.

In addition, Retired Army Lt. Gen. Keith Kellogg, who has been serving as acting national security adviser, will return to his role as the National Security Council chief of staff.