HONG KONG • State-run newspapers in Vietnam and China reported in recent days that senior military officials from the two countries would hold a fence-mending gathering along a border where their militaries fought a brief but bloody war in 1979.
But Tuesday, the scheduled start of the gathering, came and went without any of the coverage in the state news media that readers in the two countries had expected.
The Chinese Defence Ministry later said in a terse statement it had cancelled the event "for reasons related to working arrangements".
Analysts, citing government sources, said that the Chinese delegation had unexpectedly cut short a trip to Vietnam after tempers flared during a closed-door discussion on disputed territories in the South China Sea.
The cancellation is highly unusual for the two communist neighbours, and it comes as Beijing continues to build artificial islands in the South China Sea, where the Chinese seek to expand their military influence at a time of uncertainty over United States President Donald Trump's policies in the region.
"This was not what the Vietnamese expected from a polite guest," said Dr Alexander Vuving, a Vietnam specialist at the Daniel K. Inouye Asia-Pacific Centre for Security Studies in Hawaii. "You can say both sides miscalculated," he added.
But another interpretation is that both countries are "very committed to showing the other their own resolve" on matters of territorial sovereignty.
The dispute happened during a visit to Hanoi this week by General Fan Changlong of China. It was unclear what precisely roiled his meeting with Vietnamese officials, much less whether the general's actions had been planned.
Analysts said he appeared to have been angry over Vietnam's recent efforts to promote strategic cooperation with the US and Japan.
Prime Minister Nguyen Xuan Phuc recently visited those two countries in quick succession, and the Vietnamese and Japanese coast guards conducted joint drills in the South China Sea last week focused on preventing illegal fishing.
Another reason, analysts said, could be Vietnam's apparent refusal to abandon oil and gas exploration in areas of the South China Sea that both it and Beijing claim.
Though China is Vietnam's largest trading partner and a long-time ideological ally, the neighbours have long been at odds over competing claims to rocks, islands and offshore oil and gas blocks in the South China Sea, which Vietnam calls the East Sea.
Dr Vuving said a specific source of the dispute may have been the so-called Blue Whale project, a gas-drilling venture in the South China Sea by Vietnam's state oil company, PetroVietnam, and Exxon Mobil.
The drilling site, which is expected to produce gas for power generation by 2023, is close to the disputed Paracel Islands and near the "nine-dash line" that shows expansive territorial claims on Chinese maps. The project appears to set a "very damaging precedent for China's strategy in the South China Sea", Dr Vuving said.
There were unconfirmed reports on Wednesday that China had recently deployed 40 vessels and several military transport aircraft to the area.
Dr Le Hong Hiep, a research fellow at the ISEAS - Yusof Ishak Institute in Singapore, warned that new tensions could emerge in the short term. China appears increasingly eager to stop Vietnam from growing too close to Japan and the US, he said.
"As Vietnam tries to achieve its economic growth targets, it is planning to exploit more oil from the South China Sea," Dr Hiep wrote in an e-mail. "As such, the chance for confrontation at sea may also increase."