New fighting along the Thai-Cambodian border is rooted in a longstanding territorial dispute

On Monday, the heaviest fighting since the ceasefire erupted. While it is unclear who took the first shot, Thailand launched airstrikes along the border as ground combat also broke out.

The dispute goes back to the early 20th century

Thailand and Cambodia have a history of enmity over centuries and experience periodic tensions along their land border of more than 800 kilometers (500 miles).

The competing territorial claims largely stem from a 1907 map created while Cambodia was under French colonial rule, which Thailand contends is inaccurate. Many Thais are still angered by a 1962 ruling by the International Court of Justice, which awarded sovereignty of disputed land to Cambodia — a decision reaffirmed in 2013. The disagreement fueled several armed clashes between 2008 and 2011.

The Trump-backed ceasefire was fragile

An uneasy peace was reached in late July, when Malaysia pushed for peace talks and President Trump brought the contending sides to the negotiating table by leveraging the importance of the U.S. market for both nations' exports, threatening to withhold crucial trade privileges.

Trump afterward claimed this intervention as one example among several from around the world where his actions led to peace between warring nations.

The preliminary pact was followed by a more detailed October agreement. Its terms called for coordination of de-mining operations, removal of heavy weapons and equipment from the border, implementation of measures to restore mutual trust, and desisting from harmful rhetoric and the dissemination of false information. None of these actions were implemented in full, if at all.

Both nations continued fighting a bitter propaganda war and there have been occasional minor outbreaks of cross-border violence.

A major Cambodian complaint has been that Thailand continues to hold 18 troops taken prisoner. Thailand accuses Cambodia of laying new land mines in the areas under dispute that maimed Thai soldiers. Cambodia says the mines are left over from decades of civil war that ended in 1999.

The failure to implement the ceasefire terms was used by the Thai side as an excuse not to promptly release the Cambodian prisoners, even though the October agreement urged it to do so "as a demonstration of Thailand's desire to promote mutual confidence and trust."

The conflict impacts diplomacy, trade and tourism

Thailand is one of Washington's closest and most longstanding allies. The country also holds a huge military advantage, best demonstrated by its mostly unchallenged ability to use air power.

But Cambodia has also been trying to strengthen its hand diplomatically. It was among the first countries to strongly support a Nobel Peace Prize nomination for Trump, even bringing out crowds to demonstrate in favor of that.

Cambodia has also employed an intensive propaganda campaign on social media portraying itself as the underdog and issuing frequent unverifiable accusations about Thai actions. Nationalistic saber-rattling has been ubiquitous on both sides.

Washington for its part appears to be trying to actively build better relations with Cambodia to woo it away from its close relationship with China, but that in turn has bred resentment in Thailand.

Cambodia has made further progress toward finalizing a trade deal with Washington than has Thailand, whose economy is much bigger and more complex.

Potential economic consequences go beyond trade.

The renewed fighting comes just as the winter tourism season is hitting its peak, and risks deterring tourists. Tourism is a major earner for both nations, which are still trying to recover from the battering the industry took during the coronavirus pandemic.

Cultural competition is also a factor

The ill feeling between the two neighbors is not just about overlapping border claims, but also deep-seated cultural enmity that has its roots from centuries ago, when they were large and competing empires.

In more modern times, bad feelings have lingered, as Cambodia's development, hindered by French colonialism and, in the 1970s, the brutal rule of the communist Khmer Rouge, has fallen well behind Thailand.

Both have fought over claims on cultural products ranging from boxing, mask dancing, traditional clothing and food.