December 1937. Japan's advance across China was in full swing. Manchuria was the first to fall, then Shanghai. It was only a matter of time before Japanese forces reached Nanjing.  

They arrived at the city gates on December 9, with a simple message: surrender or there will be no mercy. 

Nanjing was the capital of the Republic of China, the seat of government. But leader Chiang Kai-Shek and his Nationalist government had fled elsewhere. Nanjing was alone. And unprotected.

Apart from one man. John Rabe, a German national, mustered Chinese troops and set up safe zones in embassies around the city. It's believed he saved a quarter of a million lives.

READ MORE John Rabe, the German businessman who saved thousands of lives

But on December 10, the 6th and 116th division of the Japanese Army breached the city walls. Within two days, Chinese troops had retreated. Nanjing, like Shanghai a few months before, had fallen.

What followed will be etched on the minds of the Chinese people for generations. Japanese troops were ordered to inflict maximum pain on the people and to show no mercy. It was an instruction they followed to the letter.  

During the tribunals that followed, missionary John Magee offered sobering eye-witness testimony.

"The killing started immediately in several ways," he told the court. "Often by individual Japanese soldiers or up to 30 soldiers going about together, each one seeming to have the power of life and death. And then soon, there was organized killings of great bodies of men."

It was cruelty for cruelty's sake. In the space of six weeks, 300,000 were dead. Most killed in unimaginably brutal ways. For China, a humiliation unparalleled.

But the horrors of that winter largely went unrecognized in the wider world. Professor Andrew Field, from Kunshan Duke University, says the massacre was poorly understood in the West.  

"I think partly because China was so far away," Field explained to CGTN. "This was before World War II really started in Europe. So the reports of a war in Asia would have felt kind of distant in, say, Europe and America. 

"But there were eyewitnesses, there were many Westerners who were living and working in Nanjing at that time, who became important eyewitnesses as well as contributing or at least trying to contribute to the safety of the Chinese people in Nanjing."

Those records are kept in the Nanjing Museum, along with a wall of the dead. Names etched in stone. Here too, the pictures of those who somehow escaped; who lived through it, but carried the horror with them ever since.  

It's a horror that is still not acknowledged by Japan to this day. Without that apology, healing is hard.  

The Rape of Nanjing, as it's known, ended as quickly as it began. Japanese troops were called away to fight elsewhere. 

They left behind a city destroyed, its people brutally murdered, and China nursing a wound that its people will carry in their collective psyche for generations to come.