Key point: South Korea is taking it seriously.

The North Korean People’s Air Force is predominantly composed of Soviet jet designs from the 1950s, or at least their Chinese clones, plus a smaller number of aircraft from the 1970s and 1980s. However, the most numerous aircraft in North Korean service is even more antiquated: a single-engine biplane transport that first flew in 1947. North Korea operates over three hundred An-2 and Y-5 biplane transports in at least six regiments, and they play a key role in its military strategy.

How could these chunky biplanes pose a threat to adversaries fielding guided missiles and supersonic stealth fighters? North Korea has a found a way, banking on its own experience in the Korean War.

Even when the first An-2 prototype was unveiled in the Soviet Union in 1947, its biplane configuration seemed anachronistic. Aircraft with two wings exhibit more lift, lower stall speeds and superior maneuverability—but the extra drag created by the second wing imposes steep limitations on maximum speed. This led biplane designs to rapidly fall into obsolescence by the start of World War II.

However, renowned air transport designer Oleg Antonov had appreciated how slow two-seat Po-2 utility biplanes had proven highly successful at harassing German troops at night, and landing supplies and secret agents behind enemy lines; they were difficult for fast fighters to detect and intercept, and their excellent low-speed handling allowed them to operate from very short, unprepared fields. Antonov’s idea was to basically scale the Po-2 up to a light transport for agricultural and civilian use from primitive airfields across the USSR’s vast, lightly populated frontiers.

The basic An-2 has a single, throaty one-thousand-horsepower ASh-62r radial engine that rattles like a tractor. This can push the fabric-and-aluminum aircraft to maximum speeds of little more than 160 miles per hour and out to a range of 560 miles. Its cargo compartment can accommodate up to twelve passengers, or between one or two tons of cargo. Though a bit of a tail dragger, the An-2 is a remarkably forgiving aircraft, as the added lift from its biplane configuration and two sets of full-wing length flaps means it can operate at speeds as slow as thirty miles per hour without stalling.

In fact, if flown into a sufficiently strong headwind, an An-2 can almost hover in place, or even briefly fly backwards. These low-speed characteristics give An-2s an almost helicopter-like ability to take off or land from short and very rough, unprepared airfields, as you can see in this video. Of course, helicopters are even more flexible and fly at comparable speeds, but they are also considerably more expensive and maintenance-intensive. An-2s on the market typically cost between $30,000 and $100,000.

Soviet economic and military planners caught on to the An-2’s potential after Stalin’s death and it became a global hit, with exports to more than fifty countries and production eventually moving to Poland. China also license-built thousands of An-2 clones, called the Y-5. More than eighteen thousand of the pokey biplanes were eventually built, with Polish factories not ending production until 2001. China is still building Y-5s, and I have frequently observed the biplanes overflying dusty rural regions in northern China.

Although most renowned for civilian service as passenger liners, air ambulances and crop dusters—the An-2 was nicknamed Kukuruznik (“Corn Eater”)—the An-2 also had plenty of useful military applications (NATO codenamed it “Colt”), ranging from logistics to parachute training, artillery observation and infiltrating special-operations troops. Dozens of specialized variants were developed, including An-2V floatplanes and even ski-equipped Colts for arctic operations.

By one count, An-2s were used in over forty conflicts, primarily in transport, observation and occasionally light attack roles. A quartet of North Vietnamese An-2s once attacked a secret U.S. base in Laos with rockets and mortar shells dropped out a side door—though one was shot down by small-arms fire from a Huey helicopter. In 1991 the newborn Croatian Air Force used an An-2 to ferry supplies to the besieged city of Vukovar, and even attacked Serb forces using barrel bombs made out of boilers and fuel drums.

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During the Korean War, North Korea had used Po-2 utility planes in a similar fashion as the Soviets had in World War II to launch night raids behind UN lines, which sometimes inflicted considerable damage. U.S. ground-based radars struggled to detect the slow and low-flying intruders, while sophisticated radar-equipped night fighters only succeeded in shooting down thirteen.

The KPAAF received its first An-2s before the war’s end, but it’s not clear if they saw action. However, the KPA clearly saw the potential for the An-2 to pick up where the Po-2 had left off. The An-2’s capabilities synergized especially well with the emphasis North Korea accords to infiltrating its special-forces branch—the largest in the world, numbering around two hundred thousand personnel—deep behind South Korean lines to create a “second front.”

Ground-skimming An-2s would offer a useful platform to parachute or air-land elite commandos to attack key bases and installations, disrupt supply lines and generally spread chaos. Indeed, in 2015 the KPA conducted three separate large-scale parachute exercises involving fifteen thousand commandos. The An-2s have featured prominently in state media; for example, in 2015, Korean Central News published a photo of Kim Jong-un sitting in the cockpit of a “new” An-2, which he ostensibly had tested personally.

The same year, the press proudly announced the An-2s had received a new camouflage scheme (green on top, blue on the bottom) to make them hard to distinguish from both the ground and air. In fact the An-2s have reportedly been maintained to a higher standards of readiness than the KPAF’s dilapidated fleet of jet fighters.

Wouldn’t such primitive aircraft be vulnerable to South Korean air defenses and fighters? Yes, but only to a point. The An-2 already has a reduced radar profile due to its fabric surface, which is less radar reflective. The KPAF has reportedly replaced additional aluminum surfaces on its An-2s with canvas and fabric, and even installed wooden propellers to further reduce the signature. Additionally, the An-2’s ability to hug the terrain of Korea’s famously rugged mountains would help mask it from the radars of ground-based surface-to-air missiles, and pose a challenge to airborne radars. Finally, the Kukuruznik’s slow speed may cause some radars to eliminate it from the track as background noise.

This is not to say that the An-2 is an invulnerable stealth plane. Some will inevitably be detected, and they would be exposed to any short-range air defenses, including heat-seeking shoulder-fired Stinger missiles, heavy machine guns and rapid-fire autocannons. However, surging dozens or hundreds of difficult-to-detect An-2s could easily overwhelm the air defenses on the DMZ, even if the losses sustained in doing so might be considered unacceptable to a Western military mentality.

North Korean An-2s have also been modified to fire unguided rockets, rocket-propelled grenades and bombs, perhaps to provide air support for the special operators after they have been dropped off. You can see a video from 2014 of Kim Jong-un looking on as a rocket-equipped Il-2 blasts a target on a grassy field here—followed by a similar demonstration from a Yak-18 two-seat trainer, which was employed successfully as a night intruder during the Korean War.

The Drive’s Tyler Rogoway has also suggested An-2s could serve as a delivery system for a small nuclear bomb, although this would only be possible if North Korea develops a sufficiently lightweight nuclear warhead. Furthermore, given the dicey odds of any one individual An-2 penetrating the air-defense gauntlet, such a strategy would likely require warheads redundantly deployed on multiple aircraft.

The threat of being swarmed by low-observable biplanes has apparently led South Korea to acquire two An-2s of its own, so that its F-15 fighters can practice interceptions, according to this article in NK News. The reality is that, while an An-2 infiltration swarm would be difficult to repel, it would still likely be a suicide mission for many of the pilots. However, the aircraft are expendable, costing less than most of the guided missiles that might be fired at them, and North Korean infiltrators have exhibited a willingness to sustain heavy casualties and fight to the death during their many previous misadventures.

Pyongyang’s antiquated combat aircraft have limited prospects in a head-to-head confrontation with South Korean airpower, let alone that of the United States. The KPA’s continued investments in its An-2 is an example of how it has put considerable thought and effort in finding ways for its outdated hardware to project a credible asymmetric threat.

The An-2 will remain in service for many more years in the twenty-first century, largely in peaceful civilian roles. Hopefully a bloody conflict on the Korean Peninsula will not become part of its colorful history.

Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring. This first appeared last year.