On the day of North Korea's first atomic test in 2006, aides to President George W. Bush began phoning foreign capitals to reassure allies startled by Pyongyang's feat.

The test, aides said, had been mostly a failure: a botched, 1-kiloton cry for attention from a regime that had no warheads or reliable delivery systems and would never be allowed to obtain them. "The course they are on is unacceptable," State Department spokesman Sean McCormack said publicly at the time, "and the international community [will] act."

A decade later, that confidence has all but evaporated. After a week in which Pyongyang successfully lobbed four intermediate-range missiles into the Sea of Japan, US officials are no longer seeing North Korea's weapons tests as amateurish, attention-grabbing provocations. Instead, they are viewed as evidence of a rapidly growing threat - and one that increasingly defies solution.

Over the past year, technological advances in North Korea's nuclear and missile programmes have dramatically raised the stakes. Pyongyang's growing arsenal has rattled key US allies and spurred efforts by all sides to develop new first-strike capabilities, increasing the risk that a simple mistake could trigger a devastating regional war.

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The military developments are coming at a time of unusual political ferment, with a new and largely untested Administration in Washington and with South Korea's Government coping with an impeachment crisis. Longtime observers say the risk of conflict is higher than it has been in years, and likely to rise further as North Korean leader Kim Jong Un seeks to fulfill his pledge to field long-range missiles capable of striking US cities.

"This is no longer about a lonely dictator crying for attention or demanding negotiations," said Victor Cha, a former adviser on North Korea to the Bush Administration. "This is a now a military testing programme to acquire a proven capability."

North Korea began building its first reactor for making plutonium more than three decades ago. It has shown ingenuity in increasing the range and power of a stockpile of homemade short- and medium-range missiles, all based on Soviet-era designs.

Often, in the past, the new innovations have been accompanied by demands: a clamouring for security guarantees and international respect by a paranoid and nearly friendless Government that perceives its democratic neighbours as plotting its destruction. After the first atomic test in 2006, then-leader Kim Jong Il threatened to launch nuclear missiles unless Washington agreed to face-to-face talks.

North Korea has been slammed instead with ever-tighter United Nations sanctions meant to cut off access to technology and foreign cash flows. Yet, in spite of the trade restrictions, diplomatic isolation, threats and occasional sabotage, the country's weapons programmes have continued their upward march, goaded forward by a succession of dictators willing to sacrifice their citizens' well-being to grow the country's military might.

Pyongyang's fifth and latest nuclear weapons test occurred on September 9. Scientific analyses of the test determined that the new bomb's explosive yield approached 30 kilotons, two times the force of the "Little Boy" bomb dropped on Hiroshima, Japan, in 1945. The device was twice as powerful as the bomb North Korea tested just nine months earlier, and 30 times stronger than one detonated in 2006 in a remote mountain tunnel. More ominously, North Korea last March displayed a new compact bomb design, one that appears small enough to fit inside the nose cone of one of its missiles.

Regardless of whether the miniature bomb is real or a clever prop, North Korea does finally appear to be "on the verge of a nuclear breakout," said Robert Litwak, an expert on nuclear proliferation . He said Pyongyang's arsenal is believed to now contain as many as 20 nuclear bombs, along with enough plutonium and highly enriched uranium to make dozens more. "When I got into this field," Litwak said, "I couldn't have conceived of North Korea acquiring a nuclear arsenal approaching half the size of Great Britain's."

The country's missiles also have grown more sophisticated. Last year, North Korea's military conducted the first test of a two-stage ballistic missile that uses solid fuel - a significant advance over the country's existing liquid-fuelled rockets because they can be moved easily and launched quickly. Also in 2016, North Korea broadcast images of engineers testing engines for a new class of advanced missiles with true intercontinental range.

US President Donald Trump, just before taking office, appeared to taunt Pyongyang on Twitter, saying that North Korea's plan for building intercontinental ballistic missiles "won't happen". A month later, Kim Jong Un launched one of the country's new solid-fuel missiles.

Last week's coordinated launch of four intermediate-range missiles appeared intended to showcase the country's ability to fire multiple rockets simultaneously at US military bases in Japan, increasingly the likelihood that some will penetrate anti-missile shields.

There have been notable failures as well. Numerous test rockets have drifted far off course, and others never made it off the launch pad. Many analysts believe it could still be several years before Kim can construct a true ICBM that could reliably reach the US mainland, and perhaps longer before he can demonstrate an ability to incorporate a nuclear payload into his rocket design. Yet, already, the basic components for a future arsenal of long-range, nuclear tipped missiles already are in place, said Jeffrey Lewis, an expert on North Korean weapons systems.

"The ICBM programme is real," Lewis said. "They've showed us their static engine test. They showed us the mock-up of the nuclear warhead. They have done everything short of actually testing the ICBM. When they do test it, the first time it will probably fail. But eventually it will work. And when it works people are going to freak out."

North Korea's state-run media have occasionally shown propaganda footage of Kim huddling with his generals over what some analysts have jokingly called the "map of death": a chart that portrays Japanese and US mainland cities as potential targets.

The laughter has now stopped, said Lewis. "This idea that these things were just bargaining chips - something that was true years ago - is superseded by the fact that there is now a rocket force ... with a commander and a headquarters and subordinate bases, all with missiles. This is now a living, breathing thing."