North Korea has indeed conducted its sixth nuclear test. The U.S. Geological Service and the China Earthquake Administration recorded a 6.3 magnitude earthquake as a result of the detonation on Sept. 3, followed by a 4.1 magnitude quake due to a suspected cave collapse resulting from the explosion. It's unclear at this time how much radiation and other radionuclide (unstable atoms released with excess nuclear energy) indicators have escaped into the atmosphere, but U.S. and Japanese aircraft specialized in tracking nuclear radiation are already airborne. Numerous ground-based Comprehensive Nuclear-Test-Ban Treaty monitoring stations are attempting to collect sample data to further assess the test as well.
So far, all data points to a much larger explosion than any previous North Korean nuclear test. Based on the magnitude of the explosion, the yield of the device could easily surpass the 100 kiloton-level, underlying the strong likelihood that a much more powerful hydrogen bomb was tested.
The North Korean government in fact claimed that they had successfully tested a hydrogen bomb. It also said that the hydrogen bomb tested was specifically designed to fit into an intercontinental ballistic missile (ICBM). Prior to the test, North Korean leader Kim Jong Un and other officials were shown standing around what appeared to be a model of a miniaturized hydrogen device resembling a two-stage thermonuclear bomb. Finally, Pyongyang claimed that the device was yield-adjustable, or that it can be preprogrammed to detonate at variable explosive power levels. This implies that North Korea could mass produce a single design for multiple functions, and that they could have tested the bomb at a much higher yield setting. It's possible, then, that the North Koreans deliberately did not detonate a hydrogen bomb at a higher yield level setting to avoid collateral damage and emissions.
The United States may now calculate that North Korea cannot be deterred or that the risk of deterrence failing is too high. Washington could also determine that allowing North Korea to develop ICBMs is unacceptable because of the risk of nuclear proliferation. In such a case, the United States would resort to preventive military action to disarm the country.
Regardless, the United States is running out of time to stage such a preventative attack. Washington is already constrained by the technological sophistication of North Korea's missiles and warheads — mainly their ability to effectively reach, reliably detonate at, and significantly damage targets in the continental United States. The uncertain number of such missiles believed to be in North Korea's arsenal is equally problematic: The United States is more likely to risk a first strike to eliminate a few North Korean missiles, not a large arsenal. Although the U.S. intelligence assessment on the projected size of North Korea's ICBM arsenal is unclear, North Korea is already estimated to be on track to field a reliable one by the end of 2018. A powerful thermonuclear warhead would shorten the window for U.S. intervention further, as an ICBM equipped with a thermonuclear warhead doesn't need to be precise or possess reliable reentry technology to be a threat, enabling North Korea to rely on the weapon's sheer destructive power to compensate for deficiencies in other missile development areas. Even a high-altitude airburst with a megaton warhead would cause severe collateral damage and fallout across large swaths of the United States. The latest North Korean test, especially if confirmed as a thermonuclear one, could boost North Korea's ability to deter U.S. and allied military action.
The latest nuclear test would also appear to reduce the chance for dialogue with the North, despite past calls by China, Russia and South Korea to pursue engagement rather than isolation. South Korea has already said it will seek tighter U.N. sanctions to isolate North Korea, and the U.S. Treasury Department said it intends to pursue sanctions designed to cut all North Korean trade. China will continue urging caution to avoid escalating into a potential military scenario, but has argued privately that "crippling" sanctions against the North will only hurt the people of North Korea, not stop the government's pursuit of nuclear weapons and long-range missiles.
The latest test has solidified the positions of all the countries involved, and made the rift between Washington and Seoul even more apparent. South Korean President Moon Jae In has sought a policy of containment and engagement with North Korea, something U.S. President Donald Trump criticized in a tweet following the North's nuclear test. While U.S. and South Korean security officials held phone calls, it was reported that Trump would be calling Japanese Prime Minister Shinzo Abe, not the South Korean president. The strain between the allies leaves more room for North Korea to exploit, and highlights the concerns in Seoul that the United States may ultimately pursue a military path, despite the South's objections.