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It’s a bit of a mystery: Why has North Korea stopped testing missiles?

Pyongyang’s last test launch was in mid-September. Prior to that they tested at a rapid pace – seven in 2017’s third quarter. Since then, zip, zero, nada. The launch pads have been quiet. North Korean state media haven’t released new photos of leader Kim Jong-un grinning or clapping as a new missile roars into the sky.

It’s tempting to speculate about possible geopolitical reasons for this interregnum. Perhaps President Trump’s rhetoric, such as his vow to use “fire and fury” to counter North Korean nuclear threats, has given Pyongyang pause. Perhaps economic sanctions are biting. Perhaps China has leaned on North Korea – finally – to rein in its impetuous behavior.

Perhaps – but it’s more likely the pause has practical causes, say experts. In recent years North Korea has generally tested few missiles, if any, in the fourth quarter of the year.

“North Korea tests its missiles when it’s ready to. They’ve got a program in place that probably has a schedule and a timetable for deliverables,” says Shea Cotton, a research associate at the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies at Monterey, in an email response to a reporter’s inquiries.

Mr. Cotton is an expert in open-source intelligence who has built and runs a North Korean Missile Test Database. He is also part of a team that uses satellite images and modeling techniques to geo-locate North Korea’s missile-test and nuclear sites, and even the exact location of Mr. Kim's propaganda photos.

His data show that North Korea accelerated its missile test program beginning in 2014. It stepped back a bit the next year, then leapt ahead in 2016 with 24 major tests.

So far in 2017 there have been 19 tests, according to Cotton’s figures. There have been none, so far, in the year’s fourth quarter.

It’s possible that may soon change. The Japanese government on Monday told reporters it had detected radio signals indicating North Korea might be preparing a test launch. The signals were not definitive, according to Japanese media.

Asked about this development, Pentagon spokesman Col. Robert Manning said the US was watching the situation carefully.

“The Republic of Korea and US alliance remains strong and capable of countering any North Korean provocations or attacks,” the colonel told US reporters.

But a drop-off beginning in September is common for Pyongyang. The pace for the first three quarters of the year is roughly similar, ranging from an average of 4.1 to 4.8 since 2012. The average for the quarter that stretches the last three months of the year, however, is 0.8.

The consistency of this quiet period supports the idea that it is caused by circumstances that recur every year. Cotton believes one obvious and prosaic reason may be the cycle of agriculture. Harvest time starts every year in September. In North Korea, scarce personnel and fuel resources may be diverted to getting in the crops. Fields often surround military bases, for example. Troops may be used for harvest purposes.

In this context North Korean tests appear less planned provocations than part of a plan for a steady march towards greater and greater ballistic missile capabilities. It is not impossible that some move on the part of the US and its allies could produce a North Korean response. Mr. Trump’s relisting of North Korea as a state that backs international terrorism could well irritate Kim, for instance.

But it would have to be a final straw, something that adds to accumulated grievances and produces action, says Cotton.

“I could see the regime doing a missile test or two before the end of the year, but I don’t see them returning to the rapid pace of testing missiles every other week like we saw earlier this year,” he says. “That’ll probably have to wait until next February or March.”

February might be the preferred month, other experts point out. That’s when South Korea will host the Winter Olympics, in Pyeongchang, only about 60 miles from the border between the Koreas.

The Olympics could showcase South Korea’s economic progress and social stability, relative to its northern neighbor. Lobbing a multi-stage missile or two into the ocean might be a way for Pyongyang to crash this international party of sport.

Meanwhile, the US appears to be intent on irritating the North, rather than pulling together a comprehensive plan that might actually change its behavior, according to Anthony Cordesman, the Arleigh A. Burke Chair in Strategy at the Center for Strategic and International Studies in Washington.

Relisting North Korea as a terrorism sponsor is a case in point, says Mr. Cordesman. It effectively ratchets up tensions with Pyongyang without doing anything to affect the underlying situation. Sure, it grabs headlines in the US – but it also gives Kim a reason to reply in kind.

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“I think the problem is [the relisting] was done without any regard to a consistent overall strategy,” says Cordesman.

As the steady progress of missile tests shows, the reality is that it may be too late to stop the march of North Korea’s development of nuclear weaponry, according to Cordesman. The only viable US option may be a strategy of containment, which accepts the reality of North Korea’s new power and tries to counter it directly, as the US did with the USSR during the Cold War.