Last year, Thailand and Cambodia fought two brief conflicts over their 800-kilometre-long land border that killed dozens of people and forced a half million people to flee their homes.
Now, tensions are escalating again between the neighbours, this time over their maritime border.
Earlier this month, Thailand terminated a 2001 memorandum of understanding (MOU) with Cambodia concerning their maritime disputes.
In response, Cambodian Prime Minister Hun Manet indicated he would initiate a dispute resolution process under the United Nations Convention on the Law of the Sea (UNCLOS).
The only previous time this process, known as "compulsory conciliation", has been used was in the Timor Sea dispute between Australia and Timor-Leste in 2016-17.
What's behind this new dispute and where could it be heading?
What's at stake?
Cambodia and Thailand both claim a large overlapping maritime area in the Gulf of Thailand. This is due to different interpretations of how their maritime boundaries should connect from the end of the land boundary on the coast to the central part of the Gulf of Thailand.
Cambodia's line results from a questionable interpretation of a 1907 land boundary treaty between France (Cambodia's colonial ruler) and Siam (present-day Thailand). This line cuts across the Thai island of Koh Kut (or Koh Kood).
Meanwhile, Thailand's claim ignores Cambodia's coastal islands.
While the disputed area is important for fisheries, there's a bigger potential prize: the untapped reserves of oil and gas believed to be in the Pattani Basin. These potentially lucrative resources have not been developed due to the dispute.
In order to unlock the reserves, Cambodia and Thailand signed the MOU in 2001. It functioned like "an agreement to agree". Both sides pledged to seek agreement on a boundary line in the northern part of the disputed area and work towards a deal on joint development in the southern part.
But now Thailand has cancelled the MOU due to what it calls a lack of progress in negotiations and the repeated conflicts with Cambodia over the last 25 years.
In January of this year, Cambodia's parliament became the last Southeast Asian country to ratify UNCLOS , the main international legal framework governing the world's seas. This opened the path for Phnom Penh to now use compulsory conciliation in the dispute.
What happened in the Timor Sea case?
Compulsory conciliation was pioneered by Australia and Timor-Leste in their Timor Sea dispute, which led to a maritime border treaty in 2018.
Australia and Timor-Leste's former ruler, Indonesia, had previously signed a treaty allowing for oil and gas fields to be developed in the so-called "Timor Gap" between the countries, with revenues shared equally. This agreement came to an end with Timor-Leste's independence in 2002.
A new treaty with Australia gave Timor-Leste more generous terms (90% of the revenues from resources exploited in the joint area), but not the permanent maritime boundary desired by Timor-Leste. This led to increasingly acrimonious disputes. So, Dili started the dispute resolution process under UNCLOS.
Australia at first challenged the competence of the conciliation commission, but was unsuccessful. After the negotiations began, though, Australia participated in good faith. This was critical, as conciliation can only be successful if the parties show a willingness to engage rather than retreat to inflexible positions.
Another key component to the success of the negotiations was the choice of highly proactive and creative conciliators, who gained the trust of both countries.
Their first priority was to reset the bilateral relationship by holding separate, confidential meetings with the parties to better understand their positions. This led to confidence-building measures that caused both sides to de-escalate.
Another important factor: the process had a tight deadline, initially set at 12 months (though later extended to 18 months).
The conciliators were also able to defer particularly contentious issues, such as the destination of the pipeline from the Greater Sunrise gas field and how the field would be developed. This helped facilitate agreement on a maritime boundary, though key issues surrounding the Greater Sunrise development remain unresolved.
And though the conciliation commission's final report was non-binding, it did lead to a peaceful outcome and permanent maritime boundary .
What happens next?
While Australia and Timor-Leste's bilateral relationship was at a very low point before the process began, they were not engaged in violent conflict as Cambodia and Thailand have been.
As such, Thailand may not accept the conciliation process. It may also feel dragged into an unexpected and unfamiliar dispute resolution process by a neighbour that only joined UNCLOS very recently .
Cambodia also tried to take Thailand to the International Court of Justice to resolve their border conflict last year, but Thailand rejected that option .
For Thailand, rejecting another dispute resolution process may send the wrong message to the world - that Thailand is not fully committed to a rules-based international order. Instead, Thailand could seize the opportunity to reset the relationship, resolve longstanding disputes and access valuable seabed resources.
Ultimately, the conciliation process, while compulsory, only delivers a non-binding outcome. Despite this limitation, conciliation can play an important role in helping both sides rebuild their relationship, as the Timor Sea negotiations show. It can also provide a vital road map towards dispute resolution.
No matter how this plays out, Cambodia's decision to turn to dispute resolution instead of going to war is significant. In a world where "might" increasingly is seen as "right", diplomacy may still have a chance.
Clive Schofield has received relevant funding from the Australian Research Council.
Rebecca Strating receives funding from the Department of Foreign Affairs and Trade and Department of Defence.
I Made Andi Arsana does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
