Nguyen Thanh Bien winced as he rubbed his side, turning toward a portrait of Ho Chi Minh in a living room filled with conch shells. He said he was still dealing with internal injuries two weeks after the Chinese authorities boarded his fishing boat and bashed him with iron pipes in a patch of the South China Sea claimed by both China and Vietnam.

“I got hit first in the head from behind — I was running to the front of the boat,” he said, sitting beside his father, who taught him to fish near their home on Vietnam’s south-central coast. “With the second blow, I lost consciousness.”

When he awoke, his catch, worth nearly $8,000, was gone. His ribs were broken. And three other crew members were injured.

China’s aggressive policing of disputed territory has produced the latest clash in a long, complex relationship. China ruled Vietnam for a millennium, leaving an indelible cultural mark, but Vietnam’s national identity and fierce independence spring from its resistance to Chinese empire-building, as its school students learn from a young age.

And the South China Sea is where Vietnam’s defiance is being tested — on its own and alongside other countries, including the Philippines and Indonesia, which are also struggling to hold on to parts of the sea that China seeks to control.