Who Is Gen. McMaster? Trump's New Top Security Adviser, Who Urged Army Brass to Speak Truth to Power. President Donald Trump, right, listens as Army Lt. Gen. H.R. McMaster, left, talks at Trump's Mar-a-Lago estate in Palm Beach, Fla., Monday, Feb. 20, 2017. Susan Walsh/AP. FULL TEXT: U.S. National Security Adviser Michael Flynn's resignation letter. Explained: Why Trump's National Security Adviser is in hot water over Russia call.