On September 15, a South Korean submarine successfully test-launched a domestically built ballistic missile.
That test puts South Korea into the club of now eight countries with SLBM capability and makes it the only member without nuclear weapons.
It may also open a new phase in South Korea's arms race with North Korea.
With that test, South Korea joins the US, Russia, China, Britain, France, India, and North Korea in the club of nations with SLBM capability, becoming the only member that doesn't possess nuclear weapons.
The new capability is yet another attained by South Korea's increasingly modern and sophisticated military, and it is only the latest milestone for the country's rapidly developing domestic defense industry.
It may also represent a new phase in South Korea's arms race with North Korea, which has responded with new missile tests of its own.
An exclusive club
The September 15 test, which President Moon Jae-in attended, was actually the third and final one of South Korea's SLBM program.
The first test, conducted in July, involved firing an SLBM from a submerged barge. It was followed two months later by a second "cold launch" test from the Dosan Ahn Changho, a diesel-electric sub commissioned in August.
The missiles in all tests were Hyunmoo-4-4s, a variant of the Hyunmoo-2B designed to be fired from submarines. The Hyunmoo-2B has a maximum range of 800 km, though the missile used in the third test reportedly only flew 400 km.
South Korea's government has argued that it is actually the seventh country to achieve SLBM capability, as North Korea hasn't clearly demonstrated that its active or under-development ballistic-missile subs are actually capable of launching any of its much-touted Pukguksong series of SLBMs.
South Korea's military is already considered superior to that of the North, but North Korea's growing nuclear arsenal, currently estimated to be between 67 and 116 warheads, could level the playing field in a conflict.
South Korea has invested heavily in modern, high-end military hardware, part of an effort to compensate for demographic shifts that will likely shrink the overall size of its military.
But that hardware - such as fighter jets and warships - are often in fixed locations that are known to North Korea. South Korea's missile batteries and other ground assets are also at risk of discovery by North Korean spies.
As a result, there is a huge risk that South Korea's most important military equipment could be destroyed in a preemptive nuclear attack by North Korea.
"A fundamental part of [North Korea's] doctrine is surprise," Bruce Bennett, a senior defense analyst at the RAND Corporation, told Insider. "If they are going to try and get that surprise, South Korea may get very little warning."
To better defend against such an attack, Dosan Ahn Changho-class subs, originally envisioned as cruise-missile submarines, were redesigned to carry six SLBMs. A ballistic missile could reach targets deep inside North Korea in minutes, while a cruise missile, which flies closer to the ground, could take as long as an hour depending on where it's launched.
"If they've got to preempt the North Korean preemption, they've got to have a ballistic missile," Bennett said.
Dosan Ahn Changho-class subs can also stay underwater for extended periods, giving South Korea a nearly guaranteed way to strike back if attacked.
"You can't follow the submarines," Bennett said. "It's a secure second-strike force."
'A hedge against the future'
Predictably, North Korea has not taken kindly to South Korea's SLBM development.
On September 11 and 12, it conducted a series of long-range missile tests - its first in six months - with new cruise missiles that flew 1,500 km, the maximum range of South Korea's cruise missiles.
Just hours before the scheduled launch of South Korea's SLBM, North Korea launched two ballistic missiles from train cars in the country's mountains. The missiles traveled 800 km before crashing into the into the waters of Japan's exclusive economic zone.
Pyongyang has tried to minimize South Korea's SLBM test. North Korean state media has questioned its authenticity and claimed the missile "will not be effective in war" and has "no strategic or tactical value."
On September 15, after Moon called South Korea's missile capabilities a "sure deterrence" against North Korean attacks, Kim Jong Un's sister responded by threatening the "complete destruction" of bilateral relations, describing Moon's comments as "slander and detraction."
North Korea's missile tests may be an attempt to demonstrate parity with South Korea's missile capabilities, while state media may have downplayed the SLBM test in an attempt to distract from North Korea's lack of progress on its own ballistic-missile subs.
South Korea's military said the Hwasong-8 appears to be early in development with "considerable time" needed before it could be deployed. But North Korea's growing nuclear arsenal coupled with the threat of hypersonic weapons, which are virtually impossible to intercept because of their speed and maneuverability, have only increased tensions.
If Seoul did develop its own nuclear weapons, Dosan Ahn Changho-class subs and the Hyunmoo-4-4 missiles would already be able to carry the warheads.
"If you're going to build your own nuclear weapons, what a great idea to have this submarine ready, to have the missile ready, and only have to build the nuclear warhead and put it on a missile and be set to go," Bennett said. "It's a hedge against the future."
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