PHNOM PENH, Cambodia–The white monument juts upward 30 or 40 feet, dominating the surrounding field and trees. From a distance, it looks like it might commemorate a military victory, important statesman or historical event. But this monument is different. It is filled with skulls.

On April 17, 1975, Phnom Penh fell to the Khmer Rouge. As in Vietnam, an American-backed regime corrupt, undemocratic,and leaderless collapsed in the face of determined communist nationalists. In Vietnam the result was repression and poverty. In Cambodia it was slaughter.

The Khmer Rouge, led by Pol Pot, or Brother Number One, summarily executed leaders of the old regime, emptied the cities, collectivized farmers and imprisoned much of the population. Before being ousted by a Vietnamese invasion less than four years later, the Khmer Rouge killed as many as 2 million people, an astonishing one-quarter of the population.

The number numbs. Adolf Hitler, Josef Stalin, and Mao Tse-tung each murdered more people. But none managed to slaughter one-quarter of his nation’s population.

Pol Pot’s reign of terror filled the country. As many as four of 10 residents of Phnom Penh may have died.

Some 20,000 of them were buried at Choeung Ek. Fifteen kilometers outside of Cambodia’s capital city down a country road, past shacks and homes, next to a school is land simply known as the Killing Fields.

Today, the area is dotted with large holes, mass graves (86 of 129 total) that have been excavated.

“Many holes, same, same,” explained my guide. The yield was 8,985 victims, whose skulls and bones fill the 17-level monument.

But, there’s more. Stub your toe on the path between holes and you might not find a stone. More likely, it is the tip of a leg bone or a few teeth from a jawbone poking through the dirt.

The Khmer Rouge didn’t just murder. They murdered crudely and brutally, using axes, bamboo poles, hammers and knives. Babies were simply swung against a tree. No one was immune from revolutionary “justice.”

However, Choeung Ek was the end, not the beginning. Most of those buried here started at Phnom Penh’s Tuol Sleng prison.

Tuol Sleng was a high school sitting on land scarcely bigger than a football field. In May 1976, the Khmer Rouge turned the half dozen whitewashed buildings into Security Office 21, or S‑21. Its purpose was to expose and exterminate enemies of the regime.

The Khmer Rouge were nothing if not meticulous. They kept arrest and execution records and filed confessions. They numbered and photographed incoming prisoners in profile as well as in front.

And they were busy until the end. When Vietnamese troops occupied Tuol Sleng on Jan. 7, 1979, they found 14 torture victims still in place, shackled and dead.

But it is the photos of the living now lining the walls that most haunt. Men and women. Boys and girls. Babies. Row after row of images, staring ahead.

A few are defiant, with smoldering hatred evident in their eyes. Others look bewildered. Many radiate fear, eyes wide at their inevitable fate. One seems to be crying.

Most look dead. Their hearts beat, blood flows, and nerves transmit pain, but their eyes are lifeless. Empty. Their humanity has been wrung out of them, and casually tossed aside.

Like No. 162, a man who stares vacantly into the camera. His fate was not for the fainthearted.

A stay at Tuol Sleng meant torture. The many tools are on display.

The metal bed frames and wooden slab on which inmates were shackled and beaten. The circular metal and rectangular wooden tubs in which people were drowned.

The high bar from which inmates were dangled. The box used to house scorpions loosed on prisoners. The electrical wires employed to shock the unfortunate. And the clubs, axes, hammers, shovels and knives used to punish and kill.

Although death was the ultimate goal, the Khmer Rouge carefully fenced in the cellblock balconies with barbed wire to prevent inmates from committing suicide. You would die, but only when the party decided the time was right.

If there was any justice at Tuol Sleng, it was that Khmer Rouge cadre were among the victims. This revolution, like so many revolutions before it, consumed its own. The former minister of information and the deputy foreign minister, among others, ended up at Tuol Sleng, and are now immortalized on the walls among the inmates of S‑21.

But, most of Cambodia’s dead were innocent the victims of totalitarian egalitarianism, which views individual lives as nothing and the collective as everything. Communism is not the first monumental evil to cloak itself with the rhetoric of humanity. There has been no phenomenon more monstrous, however, as evidenced by Cambodia’s Killing Fields.