US must maintain its ‘triad’ nuclear strike capability, Mattis says

US Defense Secretary James Mattis said Wednesday that his country should keep its capability to deliver a nuclear strike via the "triad", namely ICBMs, bombers and submarines, in order to maintain strategic deterrence.   
"If I want to send the most compelling message, I have been persuaded that is the triad, and its framework is the right way to go,” he said while traveling to the midwest to visit a number of locations including the headquarters of the US Strategic Command in Omaha. He said the types of weapons in each part of the triad will be reviewed.
In response to questions from reporters, Mattis declined to comment on whether North Korea detonated a hydrogen bomb earlier this month as Pyongyang claims, or whether the US would consider placing nuclear weapons in South Korea to counter the threat from the North.