After months spent slowly improving their relationship, China and Vietnam may be heading toward another faceoff. One June 20, China reportedly cut short a closed-door military discussion with Vietnam about the disputed South China Sea. The talks were intended to last through June 22, with officials hoping to continue mending fences since tensions spiked between the two countries in 2014 over China's deployment of a deep-sea oil rig near the disputed Paracel Islands. Despite attempts by both sides to downplay the canceled meeting, the situation between China and Vietnam in the South China Sea may escalate.

The two countries have been largely amicable toward each other during the past year. China softened its approach to dealing with the South China Sea's other claimants after a landmark ruling by the international Permanent Court of Arbitration. And Hanoi has recalibrated its approach as well, because of an uncertain regional commitment by the United States and instability in the Philippines, another vocal opponent to China's claim to the South China Sea. But neither China nor Vietnam have backed down on sovereignty claims, and they've made little progress managing disputes such as the arrangement between China and the Philippines near the Scarborough Shoal. China has continued to militarize its occupied and reclaimed areas. And Vietnam, despite being more subtle in its diplomatic maneuvers until recently, has also pursued its own island reclamation projects around the Spratly Islands, while developing defense partnerships with Chinese rivals such as the United States, Japan and India.

Vietnam's defense partnerships appear to have played a role in the cancellation of the June 20 meeting, with Chinese officials reportedly expressing displeasure at Vietnam's cooperation with the United States and Japan over the past two months. Vietnamese Prime Minister Nguyen Xuan Phuc visited Washington and Tokyo, meeting with President Donald Trump and making multibillion-dollar deals in Tokyo. The Tokyo visit also comes amid increased security cooperation between Japan and Vietnam, which included the transfer of Japanese coast guard and patrol vessels to Vietnam and more partnering on maritime enforcement. Hanoi's outreach fits into a long-standing strategy of partnering with external powers to counterbalance Beijing, though Vietnam has carefully avoided directly challenging its northern neighbor. But as Washington recasts its attention toward Southeast Asia, and as Japan — and to a lesser extent India and Australia — eagerly pursues a greater security presence in the area, Vietnam's regional momentum has again put it in Beijing's crosshairs. 

But the key source of contention in the June 20 meeting likely came from debates over energy exploration activities. Leading up to the meeting, Chinese military officials reportedly demanded that Vietnam abandon oil and natural gas explorations with foreign partners in the disputed areas of the South China Sea. Hanoi apparently refused the demand, leading to speculation that Beijing may once again resort to punitive measures. Energy exploration has been one of the main drivers of South China Sea tensions, because China sees any energy exploration in the disputed waters by Vietnam or the Philippines as a move to invalidate China's maritime claim. In the past, Beijing has used diplomatic pressure and maritime forces to suppress other claimants' foreign partnerships in the area. That pressure included the 2014 deployment of the deep-sea oil rig.

It's unclear what specific activities China was referring to in its most recent demand. In January, Vietnam partnered with U.S.-based ExxonMobil to develop a major natural gas project that would be close to the Paracel Islands. And Hanoi has other partnerships that are suspected to overlap with Beijing's claims, including an agreement with India's ONGC Videsh Ltd. from 2012 and the more recent involvement of Spain's Repsol. Until now, Vietnam and China appeared to have been showing restraint in the South China Sea. But as Vietnam continues to show little interest in ceasing energy exploration projects with partners other than China, it might only be a matter of time before Beijing moves against Hanoi.