Wang and Cao met in 1943 in Kunming, Yunnan Province, at a ball held by National Southwestern Associated University, which was organized by staff and students of China's most prestigious educational institutions who evacuated from enemy-occupied areas during World War II.

"My dad asked my mom for a dance and they fell in love with each other almost at first sight. That's it. That's how my father met my mother," said the younger Cao, who has been documenting this parents' memories.

During WWII, Cao was sent to the frontline in India, to act as an interpreter for U.S. Army General Joseph Stilwell, Cao said.

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The directive was so urgent that he didn't have time to even leave a message for Wang. On his way to the airport, sitting in the military truck, he saw a mutual friend of his lover and could only yell to the friend "Tell Wang Deyi I am heading for India."

"He was an interpreter, not a soldier. Certainly, he was frightened by the bullets and bombs in the real battlefield," said Cao. "He told my mom that she was the first one that came into his mind when he was desperately crouching in the damp trenches."

The only way they could keep in touch was through the military mail service.

The first letter Cao sent to Wang was a photo of himself, a young officer in uniform. Wang treasured it until the Cultural Revolution in the late 1960s when she had to destroy it because it showed Cao wearing an American military uniform. They also sent each other English love poems.

'My darling, I'm back'

At the end of the war in Asia in August 1945, the couple reunited at the railway station in Kunming, after more than one year apart. It was then that Cao proposed to Wang with a ruby ring he bought from Myanmar, saying "My darling, I'm back".

In the years since, their four children have encouraged them to do something to record their love.

When the couple retired, they revisited places they had been to and universities where they had studied. And on their Golden Anniversary in 2005, their children took them to the same place where they got married and celebrated together.

"They have been with each other for so long and the love never fades. Their relationship is so strong that even in the darkest time during the Cultural Revolution, when my dad was segregated for trial because of his service for the U.S. Army, they trusted each other and supported each other to get over the adversities," Cao said.

Cao wants more people to learn about his parents' story, as an example of how love can overcome adversity, and as a marker of modern Chinese history.

"My parents are 98 this year. Nowadays, they can barely remember many things in their life, but they can recite the love poems they wrote to each other during the wartime," he said.