An incursion of South Korean airspace by North Korean drones exposed Seoul’s lack of preparedness in defending against such threats, and it will likely take years for the military to correct its shortcomings, according to a classified U.S. intelligence assessment of the December incident.

The findings, outlined in a leak of U.S. secrets circulated on the Discord messaging platform and obtained by The Washington Post, spotlight the vulnerable state of South Korea’s air defense as its volatile neighbor’s aggressive development of a nuclear arsenal has Seoul and Washington on edge.

South Korea has prioritized its defenses to confront incoming missiles while investing heavily in growing its air and naval forces, but Seoul’s focus has come at the cost of neglecting other air defense needs, experts said — leaving the country vulnerable to a threat responsible for extensive carnage in Ukraine, Syria and elsewhere.

Five North Korean drones flew deep into South Korea on Dec. 26, including one that pierced the no-fly zone around Seoul’s presidential office. The military scrambled fighter jets and helicopters in response but failed to shoot down any of them; some of them disappeared from radar screens as they were being tracked, military officials said. The incident prompted political fallout for South Korean President Yoon Suk Yeol, who blamed the lack of preparedness on his predecessor and said he’d rush to activate a specialized anti-drone unit later this year.

Yoon’s office and the South Korean Defense Ministry declined to comment.

The Pentagon also declined to comment, saying it does not speak to “alleged intelligence leaks.” A spokesman did not address questions about whether the Defense Department believes South Korea’s vulnerabilities pose a threat to the thousands of U.S. service members and their families stationed there, or whether supplying substantial air defense systems to Ukraine has strained its ability to sell those systems to South Korea.

“The U.S. relationship with the [Republic of Korea] is stronger than ever,” Army Lt. Col Martin Meiners said, pointing to last month’s declaration of mutual defense that President Biden endorsed during Yoon’s visit to Washington.

The Discord leaks also include classified documents indicating the United States eavesdropped on South Korean officials’ private deliberations, including their consideration of a U.S. request that Seoul send artillery ammunition to Ukraine. South Korea has resisted doing so, citing the potential impact on trade with Russia and fears Moscow would retaliate by helping North Korea advance its weapons programs.

Those disclosures drew sharp backlash from Yoon’s critics, who demanded he confront Washington about the issue. Yoon’s office has said “the allegations of an eavesdropping breach are completely untrue,” but did not clarify which parts of the documents it deemed fabricated.

This latest intelligence assessment, which has not been previously disclosed, dates to early March and appears to be part of a briefing presentation for the U.S. military’s senior leadership. It sheds new light on why South Korean forces struggled to detect, track and destroy the drones during December’s incursion, pointing to an anemic air defense capability not attuned to emerging threats. Slow communication between ground radars and responding aircraft hampered the response, the document says, and South Korean commanders lacked clear rules of engagement.

The document also notes air defense coverage gaps that could be exploited by North Korean pilots looking for pockets of unsecured airspace. Concern over collateral damage was an additional factor, it says.

While Seoul has pledged to address these weaknesses and stand up a counter-drone unit later this year, it “likely will require 3-5 years to fully implement its plan” and acquire the requisite technology and armaments, U.S. officials surmise.

South Korean forces “very likely will be unable to consistently enact a coordinated response to [North Korean drone] incursions for at least the next 6 months,” the document says.

The incursion last winter was the first public incident of its kind since 2017, when a North Korean reconnaissance drone crashed near the border. The aircraft appeared to have taken photos of a new U.S. missile defense system in South Korea, Seoul said at the time.

South Korea has adopted some air defense capabilities, including its acquisition of Israeli early warning radars. The incident in December, though, was a “wake-up call,” said Ellen Kim, the deputy director of the Korea Chair at the Center for Strategic and International Studies, a Washington-based think tank.

“South Korea has been really preoccupied with those missile programs and nuclear programs” in North Korea, she said. It appears North Korea took advantage of that oversight, Kim said.

Drones have for years been used by outmatched military forces as a cost-effective way to even the odds. In Syria, for instance, Iranian-backed militants have attacked U.S. bases using one-way unmanned aircraft. In Ukraine, both Russian and Ukrainian troops use smaller drones to conduct reconnaissance and crash into targets.

Those that are smaller and fly low are difficult even for advanced air defense systems to intercept. South Korean officials have acknowledged their struggle to detect drones with a wingspan smaller than 10 feet, though they have said larger, military-size drones are easier to see.

“That’s why North Korea has been focusing on the asymmetrical capability, just like nuclear weapons,” Kim said.

Uk Yang, a military strategy expert at the Asan Institute for Policy Studies in Seoul, said South Korea lost the advantage it had after developing unmanned aircraft in the 1990s.

“The drone incursion raised questions in the presidential office about whether Seoul has been investing in its national defense in a right way,” Yang said.

The race to secure South Korea’s skies has put enormous pressure on its front-line troops, the leaked U.S. document says. Units already operate on high alert, and the newfound pressure to address failures exposed over the winter has contributed to “high levels of stress and exhaustion,” it states, warning of difficulties to come with keeping burned-out troops in uniform.

Yoon, a conservative who has taken a hard-line stance toward Pyongyang, told members of his cabinet after the incident that it had “clearly confirmed the need for more intense readiness and training.”

Kim reported from Seoul, and Lee reported from Tokyo.

previous coverage

• Air Force F-16s fly again over South Korea; crash investigation underway near Osan