In the United States, only the president can decide whether to use nuclear weapons. It’s an extraordinary instance in which Mr. Trump’s decision-making power will be absolute. He will not need to consult Congress, the courts or senior advisers on when or how to use them. He will have a free hand to craft our nation’s nuclear posture, policy and diplomacy.

On the campaign trail, Mr. Trump commented on the peril posed by the rest of the world’s growing nuclear arsenals. His return to the White House offers new opportunities for him to steer America clear of those threats. His administration will need to act urgently and with creativity, all while also demonstrating the understanding that nuclear weapons are too dangerous to be brandished as a cudgel.

The leaders of China, Russia and the United States are in the midst of a new great-power competition, a global struggle for military, economic and geopolitical dominance. But not all aspects of this contest are zero-sum, especially in nuclear weapons matters. There are ample opportunities for all sides to improve their own national security conditions by staving off a costly arms race and dangerous confrontation.