I recently read a book titled "The Fleet that had to Die" by Richard Hough. It details the journey of the Second Pacific Squadron and its fate during the Battle of Tsushima Strait in 1905. During this battle, several Russian warships surrendered to the Japanese. Admiral Rozhestvensky, commander of the Russian fleet, was taken prisoner. He had sustained grievous wounds in the fighting and was rushed to a hospital in Sasebo where Japanese physicians managed to save his life. Though the Japanese were certainly very confused to see these surrenders take place, they did not treat these captives brutally, according to the book. This struck me as quite surprising, given the infamous war crimes perpetrated by the Japanese during the later Second Sino-Japanese War and Second World War. Upon further digging, I discovered an old AskHistorians question that seems to further confirm the generous treatment provided by the Japanese for their Russian POWs during the Russo-Japanese War.
So what changed between the Russo-Japanese War and the Second World War? Why did the Japanese not treat their POWs from the Second World War with the same courtesies they provided the Russians back in 1905? Was there a significant cultural or political shift that influenced this change? Or was the Russo-Japanese War just an outlier in an otherwise established record of POW mistreatment? Thanks in advance!
This is a topic that has come up here a few times in the past. There is always more to say, of course, but while you are waiting for fresh responses to your question, you might like to check out this earlier thread, with a discussion led by u/amp1212.
Three factors changed between 1904 and 1931:
Japanese culture traditionally had a contempt for captured soldiers, going back to the days of the samurai. Over time, this spirit waxed and waned, but returned with a vengeance because of government propaganda in the 1880s and beyond. All that was required for soldiers to abuse prisoners was for the government to stop caring about good treatment, and this happened in the 1930s.
Cultural factors
Unlike in Western Europe, Japan's warrior caste enver developed a tradition of captives and ransom. While samurai and knights had similar origins as retainers surrounding powerful men, the knights eventually became a landed aristocracy while the samurai remained dependent on their daimyo for their livelihoods until the end of the Tokugawa Shogunate. As a result, any perceived poor service could literally end a samurai's career. Banned since the early Tokugawa period from pursuing most occupations outside administration, war, and bodyguard duty, samurai saw unemployment as a potentially deadly condition. As a result, manuals describing the ideal behavior of a samurai in Japan focused on loyalty unto death, and condemned surrender.
However, this spirit declined significantly by the end of Japanese isolation in 1854, and was only revived by government propaganda after 1877. In the wars that caused the fall of the Tokugawa Shogunate in the 1860s, numerous important figures surrendered and defected. One of the victors of that war, Saigo Takamori, grew disillusioned with the other genro (a clique of ex-samurai who surrounded the emperor) and revolved against them in 1877. Running low on ammunition, he ordered his men to conduct one last ditch charge, which resulted in the deaths of most of them including himself.
This event greatly inspired the Genro to revive their country's old damashii, or fighting spirit, which in Japanese culture was a pseudo-spiritual concept capable of granting victory against material odds. They instituted mandatory "spiritual training" sessions to indoctrinate their troops, and changed the training regimen to a brutal mix of long marches and extended bayonet sparring sessions. Beatings became routine in the Japanese military. All of these measures were meant to instill Yamato Damashii, or "Japanese fighting spirit", in the men. This culture of privations, brutality, and death-worship in the military made atrocities inevitable as long as the government did nothing to stop them. I say this to highlight an important point - the Japanese government for the most part didn't order their men to abuse prisoners. They simply overlooked the abuse.
The failed quest for equality
In the late 19th and early 20th centuries, the Japanese government did everything in its power to restrain their military, perhaps mroso than any other government. At a time when concentration camps were the gold standard of counterinsurgency, the Japanese were praised by Western press outlets for their remarkable restraint. This was due to a goal central to Japan's government at the time - achieving equality.
Western imperialism was justified by the rhetoric of bringing civilization and eliminating barbarism. The genro reasoned that if Japan were already civilized - indeed, if it acted as one of the most civilized countries - there would be no excuse for foreign aggression. Further, the West would see Japan as a power worthy of conquering its own colonies. Understanding that they were held to a higher bar as a nonwhite power, Japanese policymakers invented all kinds of measures to restrain their soldiers from abusing civilians and prisoners. These ranged from"regimental wives" - an early precursor to the comfort women program - to harsh discipline at the sight of any infraction. Human Bullets, the most popular Japanese memoir of the Russo-Japanese War, dripped with contempt for the "stinky" Chinese and "weak" Russians, but also noted that any abuses were swiftly punished.
Several events would lead the Japanese government to give up on the quest for equality by 1931. First, in 1895, Japan was robbed of some of its gains in China by a coalition of Germany, Russia, and France. Threatening war, the "Triple Intervention" forced Japan to return Lushun (also known as Port Arthur), which Russia subsequently took. Japan avenged this humiliation by defeating Russia in 1905, but the resulting peace brokered by Theodore Roosevelt was a dissapointment, resulting in no indemnity and less land than expected. This time, Roosevelt hadn't discriminated against the Japanese - he maintained a pro-Japanese foreign policy and hoped for Russian defeat. However, against the backdrop of the Triple Intervention, the Japanese public largely came to believe they had been swindled out of their just deserts by racist Americans.
The biggest blows to the quest for equality came between 1919 and 1921. First, Japan at the Versailles Conference had proposed a racial equality clause to the League of Nations Charter. This was strongly supported by France, Italy, and most of the minor allied powers, but opposed by Britain and America. Britain was somewhat indifferent about the issue, but was convinced to oppose it by Australian Prime Minister Billy Hughes, who proudly proclaimed that "ninety five out of one hundred Australians are totally against the idea of equality". American President Wilson, whose strongest supporters were white Southerners, was afraid that his base would perceive the clause as authorizing equality for blacks. In the voting session, most parties were in favor, and none dared to vote against the proposal, choosing to abstain instead. Despite this, Wilson demanded the clause be passed unanimously due to significant "opposition".
That same year, the Chinese public discovered the scandal of the Nishihara loans. In 1917, Japan had loaned Chinese premier Duan Qirui a large amount of money in exchange for significant concessions. When the scandal was uncovered, America threatened Japan into writing off most of the loans - a massive financial loss. In 1921, Britain insulted Japan by ending the Anglo-Japanese alliance. The British defense establishment itself was divided into two camps. One saw America as a rising threat and wanted to ally with Japan to contain the US, while the second either believed the US was too strong to contain, could be friendly to Britain in the long term, or both. Canada resolved the dispute in favor of the latter by strongly opposing the renewal of the Anglo-Japanese alliance, knowing they would be the first victims of a potential British-American conflict as advocated by the containment camp. In 1922, after considerable American pressure, Japan agreed to give the Qingdao concession - originally a German colony - to China. That same year, Japan signed the Washington Naval Treaty, which only allowed Japan 3/5 of the British and American tonnage. While in practice a strategic victory (unrestrained American shipbuilding would have considerably outstripped both Britain and Japan), against the backdrop of the other actions, this treaty too was perceived as discrimination.
These events led to an explosion in literature about race relations in 1920s Japan. Both the left and right wing of Japanese politics increasingly saw the West as hypocritical, viewed "civilization" as a dog whistle, and believed no amount of good press would lead Japan to be accepted as an equal. By the invasion of Manchuria in 1931, Japan had entirely given up on behaving well for the press, and this included indifference to the treatment of enemy prisoners.