Phnom Penh, Cambodia - Young Soeun was eight months pregnant when boy soldiers with rifles and hate in their eyes ordered her to leave home.

Gathering her two young children, she began walking and didn't stop for almost a month. When she did, Young Soeun gave birth without medical care and was sent to work in a rice field soon after.

Her baby would die two years later, and so would her husband and many other family members.

The date is April 17, 1975, and the Cambodian capital is in its death throes. Surrounded for months, starved of food, and shelled daily by communist Khmer Rouge rebels, the city is overflowing with more than a million refugees from the countryside.

Hospitals can't cope with the dying and wounded. This is the last day of Cambodia's 5-year-long civil war and the first of Pol Pot's rise to power. It is the beginning of the end.

Year Zero

Schools close. Religion is banned. Money has no value. Parents are unnecessary: The revolution will raise the nation's children. Everyone must toil to build Pol Pot's agrarian society.

Almost two million Cambodians perished, many from execution and more, like Young Soeun's daughter, from sickness and starvation during the three years, eight months and 20 days that Pol Pot ruled this country.

"I feel such pity," Young says, recounting the heartrending events that began to unfold 40 years ago today when the Khmer Rouge took power in Phnom Penh.

"I could only look at my daughter's face. I could do nothing. If we even had one tablet of medicine, she would have survived," says Young, who is now 73.

"If you said one word against [the Khmer Rouge], you would be killed."

Clearing the capital

Emptying the defeated capital of its exhausted, war-weary population was the first order of Pol Pot's new regime. No one was spared.

Patients were ejected from hospitals at gunpoint and those too ill to move were left to die in their beds or along the roadside in gurneys.

Allowed to taking nothing but what they could carry, the Khmer Rouge forced the entire population of Phnom Penh and other towns, into the countryside where there was little food and no medicine.

The exiled city people had to build their own shelters in the first weeks of Pol Pot's newly named Democratic Kampuchea, a contemptuous misnomer for one of the most ruthless regimes of the 20th century.

The first to die in Pol Pot's peasant Utopia were the civil servants, soldiers and police of the defeated regime, the educated, doctors, Buddhist monks, anyone contaminated by Western influences: singers and performers.

The list was arbitrary and exhaustive.

"They were boys, some perhaps 12 years old, hardly taller than their tightly held AK-47 rifles. Their ignorance and fanaticism made them super-deadly," journalist Jon Swain wrote of the first Khmer Rouge troops he witnessed entering Phnom Penh on April 17, 1975.

Five days earlier, the US ambassador, remaining diplomatic staff, their Cambodian dependents, and others who had been offered a seat out, left Phnom Penh on Marine Corps helicopters in a mission code-named "Operation Eagle Pull".

Cambodia had joined the US in its disastrous war against Vietnamese communist forces and their protégés, the Khmer Rouge. US planes had pulverised the countryside and it population for years in a massive bombing campaign to destroy communist bases. Now, it was leaving Cambodia to suffer the terrible consequences of that alliance.

Indispensable China

The US' involvement and ignoble retreat from Cambodia is well-documented.

Less well know is China's entry into Cambodia in the days after the Khmer Rouge took power.

As the US evacuation helicopters were leaving the decks of US navy ships off the Cambodia coast in April 1975, Chinese cargo ships were waiting to unload supplies for the imminently victorious Khmer Rouge.

"China was indispensable for Khmer Rouge survival," says Andrew Mertha, author of the recently published "Brothers in Arms: Chinese Aid to the Khmer Rouge, 1975-1979."

"After 1975, China was by far Democratic Kampuchea's largest source of economic and military aid and technical assistance. North Korea was a distant second," he said.

According to Mertha, two months after the fall of Phnom Penh, Pol Pot met secretly with Mao Zedong in China and secured $1bn in military and non-military development aid. At that time, it was China's largest-ever aid pledge.

Its dealings with Pol Pot are something that China would rather forget or, at least, airbrush over. Commenting on relations with the Khmer Rouge, China's then-Ambassador to Cambodia Zhang Jinfeng claimed in 2010 that Chinese aid to the regime consisted only of "food, hoes and scythes".

"The Chinese government never took part in or intervened into the politics of Democratic Kampuchea," the ambassador was reported to have said.

Without Chinese support the regime "would almost certainly have collapsed", writes Mertha, noting that China might have been able to moderate the excesses of the regime, but did not do so.

"Instead, Beijing propped up the DK [Democratic Kampuchea] regime until the very last days of its existence, extending its reign and indirectly contributing to the vast human rights abuses and mass killings that became the defining feature of DK," Mertha writes in his book.

Managing history

Few have clean hands in this dark chapter of history.

In 1979, after Pol Pot was toppled by Vietnam and his crimes well known, the US and other countries supported the Khmer Rouge being granted Cambodia's seat at the United Nations, making them the officially recognised government of the country.

Fresh from its defeat in Vietnam, it was Washington's cynical way of punishing Hanoi for its temerity to intervene in Cambodia.

Four decades later it is remarkable how distant the Khmer Rouge regime now seems.

Phnom Penh's skyline is being redrawn with skyscrapers and its boulevards are choked with new cars and motorcycles.

After 18 years of peace and more than a decade of strong economic growth, the government of Prime Minister Hun Sen is seeking to rebrand the country, even getting advice from Coca-Cola on how to take minds off the country's history of genocide and killing fields.

A war crimes tribunal has prosecuted three former leaders of the regime and efforts are being made to bring more suspects to justice though the government, some of whom were former Khmer Rouge members, including Hun Sen, are adamantly against the net being cast wider than the three leaders already in jail.

China, too, is once again centre stage in Cambodia, providing the government with largess in the form of billions of dollars in loans and grants - far more than any other country - and without preconditions regarding governance and reform that traditional aid donors had long required.

New generation

There are no official events to mark the 40th anniversary of the fall of Phnom Penh to the Khmer Rouge, it will just be another day in a week that Cambodians celebrated their New Year holidays and were focused strictly on having fun.

Recounting the events of that time, the emotion is raw for Young Soeun and one of her surviving daughters, Vannak, who was 9 years old in 1975 and remembers the long march out of Phnom Penh and being separated in a Khmer Rouge youth camp.

Vannak's eyes fill with tears but she does not allow herself to cry.

She listens to her mother's story, and tells some of her own.

Reflecting on the huge changes over the past four decades, Vannak says that young Cambodians now have little idea what the generation that survived the Khmer Rouge regime went through.

It was a time when life and death was separated by only a word.

"Young people just don't know," she said.

Follow Kevin Doyle on Twitter: @doyle_kevin