MINOT AIR FORCE BASE, N.D.- Defense Secretary Jim Mattis is delivering a subtle message with his visit to a North Dakota nuclear weapons base: America is a mature nuclear power not intimidated by threats from an upstart North Korean leader who flaunts his emerging nuclear muscle.

Mattis on Wednesday inspected a mock-up nuclear warhead, but there was no Kim Jong Un-like posing for photographs. He chatted with nuclear missile launch officers in their underground command post, but there was no talk of unleashing nuclear hell on North Korea.

Mattis cast his visit as part of an effort to ensure that the U.S. maintains the kind of nuclear firepower that convinces any potential nuclear opponent that attacking would be suicidal. T

he Pentagon chief is getting classified briefings Thursday at Strategic Command, just outside of Omaha, Nebraska.