For more than a year now, North Korea has been quickly ticking off boxes on a deadly new weapon.
An enormous engine capable of propelling a missile to North America? Check.
Specialized "grid fins" that enhance in-flight stability? Check.
A heat-shield capable of protecting a missile as it re-enters Earth's atmosphere? Check.
Guidance systems that can deliver a payload roughly where it's aimed? Check.
A flurry of engine test and missile launches by the isolated kingdom – 24 last year, 16 so far in 2017 – has delivered an extraordinary string of successes, astounding outside observers with the speed of North Korean technological advance. The latest came Thursday, when North Korea fired four shore-to-ship cruise missiles at an area recently occupied by a U.S. aircraft carrier fleet, the latest in what has, for the last month, been a weekly barrage of tests.
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As best anyone can tell, Pyongyang has yet to fire an intercontinental ballistic missile, the kind that could strike the United States and potentially be fitted with a nuclear bomb in its tip. And some of the tests have exploded in failure.
But "what they're doing right now is not just random launches of all sorts of missiles with various different ranges," said Jung Hoon Lee, an expert in North Korean nuclear history who is director of the Institute of Modern Korean Studies at Yonsei University in Seoul.
"They have specific targets in mind, whether it's South Korea or U.S. bases in Japan or Guam – and, of course, they ultimately have in mind the possibility of bringing the U.S. mainland within their target range."
That moment, experts warn, is drawing perilously near. North Korean leader Kim Jong-un now appears to have enough proven components in place that he can create a missile to threaten North America, placing within his grasp a level of weaponry sophistication that Washington has long feared, and one that has created growing worry in Ottawa as well.
"It is incumbent on us to assume that North Korea today can range the United States with an ICBM carrying a nuclear warhead," Vice-Admiral James Syring, head of the U.S. Missile Defense Agency, said this week.
Those who monitor developments in Mr. Kim's missile program agree, and say a test of that capability is likely not far off.
"They know that simply having new missiles is not scary enough, they have to prove they work in order to cause the fear necessary to deter attack," said Rick Fisher, an expert in Asian military affairs at the International Assessment and Strategy Center in Virginia.
Demonstrating an ICBM would mark the culmination of years of effort by the impoverished country to assemble a tool that can rain destruction on the United States and force change in Washington's approach to a key Asian threat, even if North Korea's technology and arsenal size remain a shadow of what the West possesses.
"Whatever ICBM they produce, it's really unlikely that it's going to be efficient. But it's probably going to be an ICBM," said Scott LaFoy, an independent imagery analyst who studies ballistic-missile technology.
North Korea first displayed an ICBM missile known in the West as KN-08 at a military parade in 2012. That design has since been the subject of intense scrutiny and speculation, with observers puzzling over its potential range, engine technology and payload.
The best current guesses are that it can fly more than 11,500 kilometres with a 500-kilogram warhead, enough to pass over Canada and incinerate neighbourhoods in New York or Washington. The risk to Canada is highlighted in the new defence policy unveiled this week by the Trudeau government, which has begun to take more careful notice of the risks in North Korea.
"The number of countries with access to ballistic-missile technology, including some with the potential to reach North America or target Canadian and allied deployed forces, has increased and is expected to grow and become more sophisticated," the policy says. "North Korea's frequent nuclear and missile tests underscore this point."
But first missile launches are difficult, and an initial test-firing is "very likely to fail," said Jeffrey Lewis, director of the East Asia Nonproliferation Program at the Middlebury Institute of International Studies at Monterey.
"The KN-08 is basically an ICBM that's created from copied Soviet components that really weren't designed for that mission," he said.
The tests this year, however, suggest North Korea is assembling components for a much more sophisticated design. The launch of the intermediate-range Hwasong-12 missile showed what looks like "an engine designed all by themselves," Mr. Lewis said, one that makes important gains in efficiency and power, and which could be adapted into an ICBM.
Meanwhile, a successful launch of the medium-range Pukguksong-2 prove North Korea has attained the ability to use solid fuel in smaller missiles, a key technological achievement that allows for lighter missiles that can be fired more rapidly, making them less vulnerable to adversaries.
Pyongyang "is likely to pursue two tracks from here. One will be longer-range solid-fuel missiles that could reach targets such as Guam or points even farther. The other will be a true ICBM capability," said Roger Cliff, an expert on Asia security issues who is a non-resident senior fellow at the Washington-based Atlantic Council.
Bringing the new capabilities into a long-range missile could still be a lengthy process. It took India 13 years to create an ICBM powered by solid fuel.
But in North Korea, "the pace of testing this year indicates that they are trying to move as quickly as possible," Mr. Cliff said.
The U.S. military has treated the threat as serious, launching its own ground-based interceptor missile last month from California in a successful test of its ability to shoot down an ICBM of the kind that might come from North Korea.
In Asia, though, that comes as little reassurance against Mr. Kim's mania for missiles.
"North Korea has always shown that they have the political will to fire at whoever gets in their way," Prof. Lee said. Increasingly, it looks as if "they have the capability to back up their rhetoric."
It amounts to a serious challenge to the rest of the world, he said.
"Why is North Korea doing this? It's trying to make sure that even the United States is unable to interfere with their way of doing things," he said.
For Mr. Kim, the ability to threaten the United States with a nuclear-tipped ICBM provides the reassurance that "North Korea can be invulnerable."