 On July 25, 1918, Acheson was commissioned as an ensign in the Naval Reserve and served with the Naval Overseas Transportation Service until he was released from active duty on December 31 of the same year.At that time, a new tradition of bright law students clerking for the U.S. Supreme Court had been begun by Supreme Court Justice Louis Brandeis, for whom Acheson clerked for two terms from 1919 to 1921.Frankfurter and Brandeis were close associates, and future Supreme Court Justice Frankfurter suggested that Brandeis take on Acheson.