Latent tensions between Thailand and Cambodia have erupted again on their shared border, causing the fragile ceasefire backed by US President Donald Trump to collapse.

At least five Thai soldiers and nine Cambodian civilians have been killed and dozens wounded in the latest clashes, which began on Monday, according to sources officials from both countries.

These are the most serious clashes since the ceasefire was agreed upon, following the deaths of dozens of people in July.

The dispute between the two countries dates back more than 100 years, to when the borders were drawn after the French occupation of Cambodia.

But a cultural rivalry that goes back centuries also plays a role.

At the heart of it all is Preah Vihear, an 11th-century Hindu temple perched atop a cliff in the Dangrek Mountains, which form the border between the two countries.

Cultural Rivalry

For centuries, power in Southeast Asia was defined by the rise and fall of empires.

From the 9th to the 15th centuries, the Khmer Empire, whose heartland was in what is now Cambodia, dominated much of the region, including large areas of present-day Thailand, Laos, and Vietnam.

But as the Khmer Empire declined, new powers emerged.

In 1431, the kingdom of Ayutthaya, the precursor to the kingdom of Siam and modern-day Thailand, invaded and conquered Angkor, the capital of the Khmer Empire, and began occupying the western territories of Cambodia.

During the following centuries, Cambodia was caught between ambitious neighbors, and its sovereignty was repeatedly challenged.

In the 19th century, a new external actor appeared: France.

France, which already controlled southern Vietnam, pressured Cambodia into accepting protectorate status in 1863.

At the beginning of the 20th century,France further consolidated its power in the region through two treaties with the Kingdom of Siam in 1904 and 1907, which stipulated that large tracts of territory were to be ceded to French Indochina. These treaties established the principle that, in the eastern sector of the Dangrek Mountains, the border should be drawn along the watershed line. However, the mapping process carried out by French surveyors resulted in ambiguities, particularly around Preah Vihear, which lies very close to the proposed watershed boundary. During World War II, Thailand regained some of the territory it had lost under the 1907 treaty, including the temple site. After the war, Thailand was required to return the territory it had gained to French control, although Thai troops re-entered the area around Preah Vihear. Vihear in 1947.

Six years later, Cambodia gained independence from France and, in 1959, filed a claim with the International Court of Justice (ICJ) to have its territorial sovereignty over the temple confirmed.

The core of the case before the ICJ was two conflicting interpretations of the border.

Cambodia relied on a French colonial map drawn from the 1904 and 1907 treaties, which could place the temple on the Cambodian side.

Thailand argued that it had never accepted this map, or, if it had, it had done so only because of the mistaken belief that the indicated border was actually the watershed line.

In 1962, the ICJ ruled in favor of Cambodia's position on the grounds that both parties had accepted and effectively used the existing border for several decades.

The temple was virtually inaccessible during the Cambodia's civil wars and the Khmer Rouge era, and it only reopened to the public in the 1990s.

However, tensions flared again in 2008 when Cambodia succeeded in having Preah Vihear listed as a UNESCO World Heritage Site.

Thailand protested, claiming that the listing had implications for sovereignty, and between 2008 and 2011 armed clashes took place along the border, leaving at least 20 dead and thousands displaced.

Cambodia again appealed to the ICJ to seek clarification of the 1962 ruling.

The court ordered the withdrawal of troops in 2012 and, in 2013, confirmed that the temple and its immediate surroundings were within Cambodian territory.

Evacuations as clashes erupted

Thailand and Cambodia have accused each other of starting the latest wave of violence,which has led to the evacuation of hundreds of thousands of people on both sides.

Thai Prime Minister Anutin Charnvirakul stated that his country “has never wanted violence,” but that it “will use the necessary means to preserve its sovereignty,” while former Cambodian leader Hun Sen accused the Thai “invaders” of provoking the reprisals.

Trump declared on Tuesday that he was going to “make a phone call” to “stop a war between two very powerful countries.”

It is unclear how this situation will play out.

Although there have been serious armed clashes in the past, these have calmed down relatively quickly.

In July, that is what BBC correspondent Jonathan Head thought would happen again.

However, he warned that, at the moment, both countries lacked leaders with the strength and confidence needed to end this confrontation.