Thao Kae and his friends were foraging for their dinner, collecting the bamboo shoots that grow in the jungle a half-hour’s walk from their remote hamlet along the Mekong River. As they dug and sifted the soil, one of the boys found a small metal sphere and brought it back to a house in the village.

“They thought it was a pétanque ball,” said Khamsing Wilaikaew, a 59-year-old farmer, referring to the French bowling game similar to bocce. “They were throwing it against the ground.”

Four decades after it was dropped from a warplane, the metal ball, an American-made cluster bomb, did what it was designed to do. Thao Kae, 8, was killed on the spot. Mr. Khamsing’s wife and a 9-year-old boy died of their injuries days later.