In recent years Vietnam has sharply reduced the amount of time in its history curriculum devoted to teaching about the war, and students no longer learn about the My Lai massacre.

Several history teachers contacted by telephone either knew nothing about the massacre, or were vaguely familiar with it.

'I have never read about the My Lai massacre, I've only heard of it,' said Dao Thi Hanh, a high school history teacher in Hanoi.

The de-emphasis on the war has coincided with Vietnam's greatly improved relations with the US, which is now its top export market.

Quoc said he was 'very sad' about the failure to teach about the massacre, and that he had objected to the shift in the history curriculum. He said high schools also did not teach students about the occupying Japanese Army's rice requisitions in 1945, which caused a famine in which millions of Vietnamese died.

'People are becoming very pragmatic now,' Quoc said. 'They don't want to mention these kinds of events for fear it will hinder relations with those countries.'