A man watches a television screen showing a news broadcast on North Korea's nuclear test at Seoul Station in Seoul on Jan. 6. (Seongjoon Cho/Bloomberg)

NORTH KOREA’S fourth nuclear test, on Wednesday, may or may not have involved, as it claimed, a hydrogen bomb, which would represent a major advance in its weapons capability. It was, however, an unambiguous demonstration that the United States, as well as China and North Korea’s other neighbors, are failing in their attempts to restrain the regime and its ambitious 30-something ruler, Kim Jong Un. Unless those outside powers can exert more influence, Asia’s rogue state is on a path to becoming a strategic threat to the United States and its allies.

Kim Jong Un was evidently eager to claim credit for detonating a hydrogen bomb in advance of his birthday this week; it would be a signal feat for his four-year-old regime. Experts doubted that North Korea managed to build a fully fledged thermonuclear warhead, which has far greater explosive power than the fission weapons it has tested before. But the explosion comes in a context of major efforts by Pyongyang to become a fully fledged nuclear power capable of directly threatening the United States.

North Korea’s technological progress has been quite uneven, and there are many unknowns. But some experts believe it may be able to grow its arsenal from eight to 12 warheads to 20 or even 40 in the next several years. It is working on mobile intercontinental missiles that could possibly reach the western United States, along with miniaturized warheads they could carry. It has even begun testing submarine-launched ballistic missiles.

Kim Jong Un has meanwhile rejected any return to negotiations on eliminating the arsenal, which broke down in 2009. He has distanced his regime from China, which was the strongest supporter of that diplomacy, and he has proved intransigent in bilateral talks on non-nuclear issues with South Korea and Japan. Ruthless in consolidating power, he could, by the time he is middle-aged, have the capacity to attempt a nuclear strike against the United States, not to mention South Korea and Japan.

The ability of the United States to directly check this threat, short of taking military action, is limited. The Obama administration will seek another U.N. Security Council resolution toughening sanctions against Pyongyang, but four such resolutions have been passed since 2006, without much effect. Congress could legislate separate U.S. sanctions like those it applied to Iran, with the aim of deterring foreign companies and banks from doing business with the regime.

North Korea said it had successfully conducted a test of a miniaturized hydrogen nuclear device on the morning of Jan. 6, in an announcement broadcast on state-run television. (Reuters)

Ultimately, however, the United States and other governments must try to persuade China to use its unique leverage. China supplies most of North Korea’s energy as well as vital supplies of other necessities. Beijing has been willing to punish North Korea for its nuclear tests, but only to a point; it has been loath to contemplate actions that could destabilize the regime. Chinese President Xi Jinping must be convinced that the costs of acting against Kim Jong Un are less than those of tolerating him.

The Obama administration has ways to tip those calculations: by elevating North Korea to the top of the bilateral agenda; by encouraging Japan and South Korea to deploy U.S. anti-missile systems; and by stepping up U.S. sanctions on those that trade with North Korea, including Chinese companies. Neglecting the problem will only guarantee that President Obama’s successor inherits a major threat.