THE MOST solemn obligations of the presidency include responsible stewardship of the nation’s nuclear weapons and attentive duty at the apex of the command and control system. This is why a reckless tweet about nuclear weapons on Thursday from President-elect Donald Trump and his comments Friday morning are distressing.

As a candidate, Mr. Trump’s statements on this subject were a welter of different ideas. At one point he suggested the United States might withdraw its nuclear umbrella from allies Japan and South Korea; at another he expressed worry about nuclear weapons being the “single greatest threat” but boasted about being unpredictable. A president should choose words carefully on this sensitive topic. Mr. Trump did not appear to do so on Thursday when he declared, “The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.” Asked to explain by MSNBC’s Mika Brzezinski, Mr. Trump replied in a phone interview that she recounted on air, “Let it be an arms race. We will outmatch them at every pass and outlast them all.”

Nuclear deterrence is not going away soon, nor are nuclear weapons. But the United States and Russia, which together hold 93 percent of the world’s nuclear bombs, have for nearly three decades been steadily reducing those stockpiles. The New START treaty between Russia and the United States is a binding pact for these lower levels. Other arms-control agreements — notably the 1987 Intermediate-Range Nuclear Forces treaty — are strained but still in force. It took years of hard striving to reduce the mountains of nuclear warheads inherited from the Cold War. Does Mr. Trump now propose to reverse course? Would this restrain others, or spur them on?

Mr. Trump’s tweet came after Russian President Vladimir Putin delivered some chest-thumping lines of his own about strengthening nuclear forces. The fact is both Russia and the United States are undergoing separate cycles of modernizing nuclear forces and delivery systems, such as planes, submarines and missiles. The modernization is essential in some respects to maintain a credible U.S. deterrent, but at what cost? Just look at how the Navy’s plan for building 12 ballistic missile submarines is threatening to crowd out funding for other ships and submarines. Does Mr. Trump want to “expand” this nuclear bow wave still more? For what mission?

The new president will, at some point, be introduced to the nuclear war plans he would have to use in the event of an emergency. He will be shown how nuclear missiles are still on launch-ready alert, ready to fly four minutes after he gives the order, as they were during the Cold War. This is a sobering moment for every commander in chief. Ronald Reagan found it distinctly unsettling. As long as nuclear weapons are a fact of life, Mr. Trump should focus on how to lessen the dangers they pose, including keeping them out of the hands of terrorists. More does not always make us safer.