North Korea appears to have conducted a sixth nuclear test shortly after noon local time on Sept. 3. Initial reports from South Korea say the quake connected to the test measured 5.6 on the Richter scale, larger than the last test, which measured a magnitude of 5.1. Subsequent reports from the U.S. Geological Survey agency said the blast reached a magnitude of 6.3, indicating a significant increase in yield from any of the North's previous nuclear tests.
The test follows Pyongyang's claim that it had completed the miniaturization of a hydrogen bomb capable of being fitted to an intercontinental ballistic missile. North Korea has repeatedly claimed perfection of hydrogen bomb technology, producing weapons that employ a more powerful fusion reaction, as opposed to the usual fission reaction of North Korea's more primitive nuclear weapons. This is an important component of the North's deterrent strategy: It feels it must showcase that it can do significant damage to a city on the U.S. mainland to change U.S. calculus on military action or attempted regime change in North Korea.
The North's 2016 test, when it claimed to have carried out a fusion reaction, was largely dismissed by international observers because of the relatively small size of the seismic wave. Pyongyang claims that it has had to carry out only small tests because it does not have the facilities or space that the United States, China or Russia had for their nuclear tests — North Korea is a small country, and claims that if it tested a larger device underground, it would create risks for the country itself.
It is still too early to determine just what Pyongyang tested, whether an enhanced traditional nuclear weapon, or some small-scale hydrogen bomb, but it is clear that the North has not slowed the pace of its nuclear and missile program despite international sanctions, condemnation and warnings from the United States. From Pyongyang's perspective, it has a narrow window in these final stages of testing to complete a program before the United States amasses the political will and international support to carry out preventive action.
If the test is confirmed as a high yield hydrogen bomb, it would mark a very significant development for North Korea's ability to deter U.S. and allied military action. The power of a hydrogen bomb means that the North Koreans would still be able to inflict significant damage to the United States without the need for their missiles to be particularly accurate. It also means that the missile could cause major damage without having to survive reentry, as a high-altitude airburst could still prove devastating, especially to electronic systems.
South Korea has already called for a special session of its own National Security Council and it is likely the United States will call for increased sanctions and isolation of the North. China and Russia, however, are likely to call for talks, even if the talks have little long-term ability to reverse the Pyongyang's path. But as Beijing argues, at least North Korea's pace of testing slows during talks, slowing the timeline for any potential crisis. But if the North Korean program is advancing at the pace it claims, it may be too late to debate over negotiations or their meaning — the North may be closer than anyone expected to having a viable nuclear deterrent.