I’m not informed on the details. Was it much worse than nazi Germany concentration camps?

Depends on your point of view.

The US segregation laws or their centuries long slavery system? The Soviet or Chinese complete disregard of human life in relation to the state?

Yes. Yes.

The poorly documented atrocities which happened across the African continent?

Probably everything short of the Belgian Congo, yes.

Every nation has its share of horrific acts in their history and some of them pine for or defend those periods of time as if a reason for glory.

Okay, but we generally look down on those who pine for or defend Nazi Concentration Camps and recognize the Holocaust as an exceptional atrocity in world history.

The same counts for the Japanese Empire in the late Interwar period and WW2.

I mean the Mongol Horde is known for having killed around 10% of the human population at the time and raped their way around Eurasia and we don’t see people reacting with shock and horror all that much.

1. The Mongols are very often cited by people to this day when conjuring up metaphors of barbarism and atrocity.

2. The idea of the Mongols as exceptionally murderous is not supported by modern histories, and is largely, in Western histories, an artifact of “Yellow Peril” racism from the 18th century AD onwards.

3. The idea of the Mongols as exceptional rapists was never anything more than bad pop history.

Now my information is incomplete and likely incorrect in parts, but I really don’t see how the Japanese horror is much worse than all the other horrors happening in the last centuries or so.

In one example on November 23, the Nanqiantou hamlet near Wuxi was set on fire, with many of its inhabitants locked within the burning houses. Two women, one a 17-year-old girl and the other pregnant, were raped repeatedly until they could not walk. Afterwards, the soldiers rammed a broom into the teenager’s vagina and stabbed her with a bayonet, then “cut open the belly of the pregnant woman and gouged out the fetus”. A crying two-year-old boy was wrestled from his mother’s arms and thrown into the flames, while the hysterically sobbing mother was bayoneted and thrown into a creek.

“We’d take all the men behind the houses and kill them with bayonets and knives. Then we’d lock up the women and children in a single house and rape them at night… Then, before we left the next morning, we’d kill all the women and children, and to top it off, we’d set fire to the houses, so that even if anyone came back, they wouldn’t have a place to live.”

Perhaps the most notorious atrocity was a killing contest between two Japanese officers as reported in the Tokyo Nichi Nichi Shimbun and the English-language Japan Advertiser. The contest—a race between the two officers to see who could kill 100 people first using only a sword—was covered much like a sporting event with regular updates on the score over a series of days.

I figured that I’d never get another chance like this, so I stabbed thirty of the damned Chinks. Climbing atop the mountain of corpses, I felt like a real devil-slayer, stabbing again and again, with all my might. ‘Ugh, ugh,’ the Chinks groaned. There were old folks as well as kids, but we killed them lock, stock, and barrel. I also borrowed a buddy’s sword and tried to decapitate some. I’ve never experienced anything so unusual.

The Straw String Gorge Massacre occurred along the banks of the Yangtze River on December 18. For most of the morning, Japanese soldiers tied the POWs’ hands together. At dusk, the soldiers divided POWs into four columns and opened fire. Unable to escape, the POWs could only scream and thrash desperately. It took an hour for the sounds of death to stop and even longer for the Japanese to bayonet each individual. The majority of the bodies were dumped directly into the Yangtze River.[75]

In many other instances, prisoners were decapitated, used for bayonet practice, or tied together, doused in gasoline and set on fire. Wounded Chinese soldiers who remained in the city were killed in their hospital beds, bayonetted, clubbed, or dragged outside and burned alive.[76]

The Japanese also extended their “search-and-destroy” operations to the Nanjing countryside. During the Battle of Nanjing, one of the Cantonese (Guangdong) armies had broken out of the Japanese encirclement and formed guerrilla bands that harassed Japanese forces whilst retreating south. In retaliation, Japanese units systemically wiped out towns and villages spread out in the outlying regions, perpetrating rapes, arson and indiscriminate massacres which “added up to an enormous number” of deaths.[77]

Japanese troops gathered 1,300 Chinese soldiers and civilians at Taiping Gate and murdered them. The victims were blown up with landmines, then doused with petrol and set on fire. The survivors were killed with bayonets.[78]