In his first nuclear-focused speech since taking over the Pentagon in February 2015, Carter implicitly rejected arguments for eliminating any element of the nuclear force or scaling back a modernization plan that some consider too costly.

With the nose of a B-52 bomber at his back, Carter told airmen that the credibility of the American nuclear arsenal is crucial to ensuring its deterrence power. That credibility, he said, is built on personal performance.

"The confidence that you're ready to respond is what stops potential adversaries from using nuclear weapons against the United States or our allies in the first place," he said.

Earlier he flew by helicopter to a Minuteman 3 missile field and was taken 85 feet underground to a launch control center where two airmen are always present — 24 hours a day, seven days a week, 365 days a year — and ready to execute a presidential order to launch. The nuclear-tipped Minuteman 3 can reach a target on the other side of the globe in about 30 minutes.

It was Carter's first visit to a nuclear weapons base as defense chief.