Nearly a millennium ago, the patch of mountainous jungle in what is now northwestern Cambodia was a thriving center of the Khmer civilization, then one of the world’s most advanced and artistic empires. Today, Oddar Meanchey Province, on the forested border with Thailand, is home to some of the poorest people in a poor country.

It is also a battlefield, as soldiers from the two Southeast Asian nations skirmish over disputed land that cuts through ancient temple complexes carved by the Khmer Empire. Clashes in recent days have claimed lives of civilians and soldiers in both countries.

Jingoism is flaring. And at the center of the conflict, largely unnoticed by the prime ministers and generals directing the border war, are a people and a shared cultural heritage that predates modern notions of nation-states. Some residents of the area have relatives on the other side of the border.

On Friday, with rocket attacks intensifying and military drones flying overhead, Chhin Sochulsa, his wife and their four children fled their home near Ta Moan Thom temple (known in Thailand as Ta Muen Thom) for an emergency encampment sheltering thousands of displaced Cambodians.

Their home is now a piece of tarp hitched to their tractor, hammocks slung for sleep. One son is sick, and there is not enough food, he said. Already in debt, as many Cambodians are because of a microfinance crisis, Mr. Chhin Sochulsa worries about the farm animals he left behind in the war zone.

“I don’t know exactly why the clash is happening,” he said. “The situation is miserable.”

The dispute over the border has simmered for decades, and control over the area has shifted through the centuries. The Khmer Empire extended over parts of what are now the countries of Cambodia, Thailand, Laos, Myanmar and Vietnam. Its influence reached southern China, too.