Rear Admiral Michael Dumont laid out the opinion on behalf of the Joint Chiefs of Staff in a letter to Congressman Ted Lieu. Lieu asked about a potential conflict and Dumont responded "The only way to 'locate and destroy - with complete certainty - all components of North Korea's nuclear weapons programs' is through a ground invasion."
The letter went on to suggest a classified briefing to discuss matters. The Joint Chiefs of Staff directly advise the president of the United States on military matters.
Source: BBC