The Chinese military planes touched down in Cambodia over three days in June, weeks before a simmering border dispute between Cambodia and Thailand exploded into war.
The aircraft were Y-20s, known as Chubby Girls in China for their wide body and ability to carry heavy cargo. They made six flights to the southwestern city of Sihanoukville, bringing rockets, artillery shells and mortars, according to Thai intelligence documents reviewed by The New York Times, a shipment that has not been previously reported.
The Chinese weapons were packed into 42 containers and stored at the nearby Ream Naval Base, the documents said. Days later, Chinese-made ammunition was moved from the base hundreds of miles to the north, to Cambodia’s contested border with Thailand, according to the documents.
Asked for comment on the Thai intelligence reports, a senior Cambodian official did not deny many of the basic details about the shipment.
Thailand and Cambodia blamed one another for starting the war, which lasted for five days in late July. Before the conflict began, the movement of arms to the border was a crucial part of Cambodia’s buildup. For months, Cambodia had been entrenching its forces along the boundary, near an ancient temple claimed by both Cambodia and Thailand. It laid new roads and constructed a military base; all those structures were visible in satellite images.
With this buildup, analysts said, Cambodia entered the standoff with a much more provocative posture toward Thailand than that it had previously taken. But both sides relied heavily on arms from the same place: China, which has cultivated close strategic and economic ties with the two Southeast Asian states.
The accounts of independent monitors generally support the conclusions of the Thai intelligence assessment, especially about the origin of some of the weapons used by Cambodia. According to Fortify Rights, a human rights group, the rockets that Cambodia used against four Thai provinces were mostly of Chinese origin. On the first day, Thai authorities said, Cambodia struck a gas station, a hospital and civilians’ homes, killing at least 13 civilians.
“The entirety of the evidence suggests that there has been a concerted decision by the Cambodian leadership in the months and years leading up to the border clashes to change the status quo along the border,” said Nathan Ruser, an analyst for the Australian Strategic Policy Institute.
A senior officer from the Thai Armed Forces contacted by The Times confirmed the authenticity of the documents, saying that the information had been gathered by an intelligence network across military branches. Two other officers confirmed that the documents had been shared internally within the armed forces. All three spoke on the condition of anonymity to discuss documents they said were classified.
In a statement, Lt. Gen. Rath Dararoth, Cambodia’s secretary of state for defense, did not dispute the basic details of the weapons shipments to his country from China but described the Thai intelligence reports as “misleading.” He said the equipment movements referred to in the documents “coincide directly” with the conclusion of Cambodia’s annual joint military exercise with the People’s Liberation Army of China.
But that exercise had ended weeks earlier, in late May. General Dararoth would not answer any more questions.
China’s Defense Ministry did not respond to a request for comment.
Cambodia’s preparation and resupply probably allowed it to prolong its fight, but Thailand was able to assert its dominance quickly with its far more advanced arsenal. Thai forces retaliated with airstrikes by F-16 jets that bombed targets in Cambodia.
By the time a cease-fire was reached five days later, at least 40 people had been killed, including civilians on both sides, and hundreds of thousands more displaced.
China’s Balancing Act
China played an active role in trying to bring about the cease-fire, but the accounts of weapons shipments complicate Beijing’s effort to cast itself as a neutral peace broker in Southeast Asia.
While Cambodia’s military budget is a fraction of Thailand’s, each country has dramatically increased spending in recent years and turned to China for weapons. Beijing now far outranks the United States as the biggest source of arms for Thailand, which is a longstanding U.S. treaty ally.
Chinese officials have publicly denied allegations in the Thai press about arming Cambodia against Thailand. In late July, a day after the fighting began, a senior Chinese military official met with the acting Thai defense attaché in Beijing. The Chinese official said that China had not provided any military equipment to Cambodia for use against Thailand since tensions between Cambodia and Thailand had begun.
The Chinese official’s publicly reported statements did not provide specific dates.
Tensions started heating up in February, after Cambodian soldiers and civilians sang the Cambodian national anthem at an ancient temple claimed by both sides. A Cambodian soldier was killed in a skirmish in May, and five Thai soldiers were maimed by land mines in July. Cambodia has blamed Thailand for starting the conflict by cutting off access to the temple.
The Thai military intelligence reports found that, from June 21 to 23, China sent nearly 700 rounds for Soviet-era BM-21 multiple barrel rocket launchers as well as Chinese-made multiple launch rocket systems — Type 90B and PHL-03. China also shipped artillery shells for the SH-1 Chinese self-propelled howitzer and artillery for Soviet-era antiaircraft machine guns, the documents said.
Over the next two days, Cambodia moved ammunition to two border provinces, Oddar Meanchey and Preah Vihear, according to the documents.
Such a salvo of deliveries would probably have had to be approved by senior Chinese leadership, analysts said.
“This level of rapid resupply was clearly not business as usual,” said Anthony Davis, a Bangkok-based analyst with Janes, a publication specializing in defense and security.
A Contested Border
The two-centuries-old temples that are at the heart of the dispute lie near a border drawn by France in the early 1900s, when it ruled Cambodia, and are claimed by both neighbors. In the 1960s, an international court ruled that the Preah Vihear temple belonged to Cambodia, but Thailand, which refers to the shrine as Phra Viharn, never accepted the decision.
Earlier this year, Cambodia appeared to be trying to bolster its position.
It built a military base just east of the Preah Vihear temple, gaining a better vantage point over Thai troops across the border. The new structure could have been used as an artillery base, said Mr. Ruser, the Australian analyst. Starting in late 2022, Cambodia also built more roads and other infrastructure in what analysts said was a concerted effort to militarize parts of the border.
“This fortification occurred in multiple sectors along the border, which rules out the idea that it could just be one commander looking to improve their tactical positioning,” said Mr. Ruser, who studied satellite imagery of the area. “This implies that it was a much more expansive order from the Cambodian military.”
General Dararoth said the new base was part of standard improvements to Cambodia’s defenses. He added that Cambodia had “no attack doctrine in place,” and that its military activities were “not directed at any neighbor or external actor.”
Hangyu Lee, a researcher at Armed Conflict Location & Event Data, a nonprofit that tracks global conflict, said, “It remains clear that Cambodia’s military buildup was significantly more proactive than Thailand’s.”
Mr. Lee said that, in contrast with Cambodia, Thailand was “largely reactive and defensive.” The Thai Army reinforced existing outposts, built supply roads, deployed artillery and armored assets and stepped up surveillance to keep pace with Cambodian activity, he said.
Cambodia calls Thailand the aggressor and has repeatedly accused Thai soldiers of encroaching on its territory. It also blames Thailand’s pro-nationalist forces for stoking the border tensions. To resolve the conflict, Cambodia has sought intervention from an independent body such as the International Court of Justice, which Thailand has rejected.
Cambodia’s Limits and Potential Calculations
Cambodia has a much weaker military than its neighbor, and it insists that it does not want war with Thailand. So why would it start sending troops and weapons to the border?
Some analysts say that Cambodia’s de facto leader, Hun Sen, wanted to shore up nationalistic support at a time of growing discontent over the economy, or that he was driven by a falling out with a former Thai prime minister, Thaksin Shinawatra, whose daughter Paetongtarn Shinawatra was Thailand’s leader at the time of the hostilities.
Others suggest that, after years of Chinese support, Mr. Hun Sen may have been confident Cambodia was in a stronger position than in past clashes.
In 2011, during the last major clash with Thailand, Cambodia quickly ran out of ammunition, according to Rahman Yaacob, a researcher on Southeast Asia’s defense policy at the Australian National University. That shortfall pushed Phnom Penh to deepen its military ties with China.
Since then China has become Cambodia’s main military backer. The two countries have conducted annual drills together regularly for the past nine years, except during the coronavirus pandemic. In 2018, China gave Cambodia more than $100 million in military aid. Chinese weapons now make up a majority of Cambodia’s weapons arsenal, according to analysts.
“They feel they are much more well equipped compared to 2011,” Mr. Rahman said. “That’s the reason this conflict was quite serious, because heavy weapons were used instead of just rifles and small-arms fire.”
Battlefield photos posted on social media also showed Cambodian soldiers with Chinese-made SHE-40 122-millimeter artillery rockets that are used with multiple rocket launcher systems.
“Everything we saw that was being put into the rocket launchers were Chinese rockets,” said Peter Bouckaert, a conflict expert at Fortify Rights, the nonprofit.
“China should look at what the Cambodian Army did during this conflict and raise their concerns about the indiscriminate use of their weapons,” Mr. Bouckaert said. “It doesn’t improve its image when it’s supplying weapons that are being used to kill civilians in other Asian countries.”
Pablo Robles contributed reporting from Seoul, Phuriphat Dejsuphong and Kittiphum Sringammuang from Bangkok, and Li You from Beijing.
Sui-Lee Wee is the Southeast Asia bureau chief for The Times, overseeing coverage of 11 countries in the region.
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