A former American intelligence official who came under intense scrutiny during a stint at the White House last year is returning to government as the national security adviser to Attorney General Jeff Sessions, according to a person familiar with the decision. The official, Ezra Cohen-Watnick, will play an important role at the Justice Department, advising Mr. Sessions on counterintelligence and counterterrorism. Mr. Cohen-Watnick is known to be hawkish on Russia and China, believing the intelligence community needs to take a more aggressive approach in countering their activities. Both countries conduct wide-ranging espionage in the United States that targets commercial and government secrets. Mr. Sessions is broadly viewed in the Justice Department as lacking expertise in the area.