Mattis would not allow reporters to watch as he met with United States forces at the Minot Air Force Base, where 150 ICBMs equipped with nuclear weapons stand ready to launch at a moment's notice.
"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".
The Trump administration is in the midst of an in-depth review of US nuclear policy. The third way to deliver nuclear weapons is by Navy submarine.
Mattis is visiting Minot in North Dakota as part of a nuclear posture review to talk to missileers and bomber crews about readiness issues and what they think should be part of it.
At Minot, Mattis toured a missile alert facility used to control ICBMs and a storage area where airmen maintain ICBM warheads. "That's how a deterrent works".
Offutt officials say Mattis will make the stop Thursday and receive briefings.
The visit will be his first trip to Strategic Command ever since being appointed Secretary of Defense.
StratCom employs nuclear, cyber, space, global strike, joint electronic warfare, missile defense and intelligence capabilities to deter aggression against the US and respond if deterrence fails. The trip was labeled as a "three-day nuclear-focused trip that comes during ongoing reviews of the US nuclear posture and ballistic missile defense".
The visits were scheduled prior to North Korea's sixth nuclear test earlier this month, but serve as an opportunity for Mattis to highlight the Pentagon's always-ready nuclear forces.