American Secretary of Defense James Mattis made the first stop in a three-day tour of America’s nuclear triad on Thursday, as he continues to review the nation’s nuclear posture and ballistic missile defense in the face of repeated threats of preemptive nuclear strikes from Kim Jong un’s North Korean regime.

Mattis already visited the submersible leg of America’s nuclear triad on August 9th, when he met with a group of sailors tasked with manning the nation’s ballistic missile submarines at Naval Base Kitsap in Washington State.  The nuclear triad is a term used to refer to the three different methods of nuclear weapon delivery employed by the United States, designed to ensure that no offensive nuclear strike could completely wipe out America’s means to return nuclear fire, and ensure the threat of “mutually assured destruction” stays intact even if taken by surprise.

We’re on the way to Minot,” the secretary told reporters traveling with him on Wednesday. “I want to see the airmen who serve out there, on alert, all the time, intercontinental ballistic missile airmen and B-52 airmen, and talk to their commander, talk to the troops, just get a feel for how it’s going and hear directly, unfiltered, their view of their mission, their readiness,” he added.

Minot Air Force Base in North Dakota is home to not just one, but two legs of America’s nuclear triad.  Nuclear capable B-52s, which have been in service under various iterations since the 1950s, stand ready for takeoff on the airstrip, with Intercontinental Ballistic Missiles, or ICBMs, tucked away beneath the surface of America’s Great Plains.  After meeting the with B-52 nuclear bomber crews, Mattis will tour the missile alert facility that controls the ICBMs, as well as the weapon storage area where airmen maintain a stockpile of nuclear warheads.