Pyongyang’s recent firings of Iskander-type ballistic missiles and multiple rocket launcher projectiles are important developments – contrary to US President Donald Trump’s dismissive comments. The tests will likely continue for a few weeks until Seoul and Washington end their current military exercises.

The tests demonstrated next-generation missile and rocket artillery systems. They also underscored North Korean leader Kim Jong Un’s  warning that he needs to see US sanctions relief by the end of the year. And finally they highlighted a little-noticed conventional arms race between the two Koreas.

Though short in range, North Korea’s Russian-designed Iskander missile is an extremely capable system. Besides being solid-fuel, it is mobile, can fly on a depressed trajectory and has the ability to maneuver in flight – features that make it harder to hit with anti-ballistic missiles. It is nuclear capable and can strike anywhere in South Korea.

North Korea has shown a surprising capability to adopt advanced military technology. It has tested and now begun deploying a 300 mm multiple rocket launcher (MRL) system with satellite-aided navigation. The system can strike targets as far south as central South Korea with great precision. The new US military base at Camp Humphreys is within range. As a result, the wisdom of concentrating 33,000 U.S. troops and civilians on the base is questionable. North Korea seemed to have alluded to this huge installation – the largest overseas U.S. base in the world – when it spoke of a “fat target” in its statement accompanying the MRL tests.

Kim Jong Un also recently visited a shipyard where construction of a new type of missile-firing submarine is nearing completion. The next technological breakthrough may be air-independent propulsion systems, which would make North Korea’s already elusive submarines much quieter and harder to detect.

Pyongyang has sought to justify its recent missile tests as a response to Seoul’s military buildup, slamming South Korean President Moon Jae-in for “double-dealing” by “talking peace” while buying American stealth fighters and resuming joint exercises. The first of Seoul’s order of 40 F-35s arrived a few months ago. The new jets will give Seoul the capability to penetrate North Korean airspace undetected, greatly enhancing its potential first-strike capability.

Seoul decided last year to buy 90 more German-Swedish Taurus air-launched cruise missiles for its F-15 fighters, bringing its total to 260. These bunker-busters can strike leadership facilities and other high-value targets from far away with pinpoint accuracy.

From Pyongyang’s perspective, fielding these new systems is a logical response to Seoul’s acquisition of advanced conventional weaponry. While Moon is known primarily for promoting reconciliation toward North Korea, he has simultaneously been pushing ahead with an ambitious modernization of South Korea’s armed forces.

Moon’s defense plan is patterned after “Defense Reform 2020,” launched in 2006 by former President Roh Moo-hyun, under whom the current president served as chief of staff. Previously, South Korea’s military structure was heavily tilted toward the army, whose troops would be expected to do much of the ground fighting in a second Korean War while the U.S. would provide air and naval support.

But the leftist Roh sought to introduce a more balanced structure among the army, navy and air force to make the country less dependent on U.S. military protection – and to keep up with Japan.

Moon’s defense push has resulted in increased spending for weapon systems for the navy and air force. In terms of the navy, this has meant creating a “blue water” fleet able to operate in international seas.

As for the air force, Seoul wants to acquire air refueling aircraft in addition to the F-35. To improve aerial surveillance of North Korea, Seoul is buying the high-altitude Global Hawk surveillance vehicle and other smaller drones.

Moon has significantly increased defense spending. Its share of the national budget is the biggest since the mid-1980s when South Korea was under military rule.

A good chunk of the money will also be going to so-called “Kill Chain” defense systems. This project involves assembling a strike force of ballistic and cruise missiles, air power and commando units to destroy North Korea’s nuclear arsenal and the Pyongyang leadership, while upgrading Seoul’s anti-missile systems. The Moon administration hopes to complete their installation by the early 2020s.

Although North Korea has a bigger military on paper, with more aircraft, tanks, artillery and warships than South Korea, most of these weapons are outdated and would be no match for Seoul’s more modern armed forces.

North Korea is worried about developments south of the Demilitarized Zone, believing that South Korea is laying the foundation for possible “decapitation strikes” to take out the North Korean leadership.

Pyongyang is probably also worried about plans to transfer wartime operational control from Washington to Seoul in a few years. Contrary to the conventional view that North Korea’s primary goal is to push the US off the peninsula, it is more likely that Pyongyang – the weaker of the two Koreas in terms of conventional forces – would favor having Washington stay for now to maintain the status quo. It is noteworthy that recent statements accompanying the North Korea missile tests have focused criticism on South Korea rather the US.

A traditional action-reaction dynamic seems to offer the best explanation for the dynamics of this inter-Korean arms race, which is also related to North Korea’s nuclear program. Pyongyang’s missile and nuclear build-up is connected to Kim’s attempt to shift to an “economy-first” policy by reducing spending on large conventional forces in favor of more cost-effective weapons systems.

Breaking this cycle of rearmament will be difficult. South Korea is acquiring advanced weapons not only to counter North Korea, but also to guard itself against potential threats from China and even Japan. In addition, Seoul is purchasing more US military equipment to appease Trump, who wants South Korea to assume a greater share of defense burden-sharing.

Rather than focusing so single-mindedly on the nuclear issue, Washington needs to address war risk in broader terms by taking into account the conventional arms race as well.

For that, Trump needs to appreciate the perceived existential threat North Korea faces. Only then can the action-reaction dynamic be pointed in a different direction – toward winding down tensions rather than escalating them.

John Merrill is a visiting scholar at the Institute of Korean Studies at George Washington University and former head of the Northeast Asia Division in the Bureau of Intelligence and Research at the US State Department. John Burton is a former Seoul bureau chief for the Financial Times and has written about Korean affairs for more than 25 years.