|Japanese soldiers approach the walls of Nanjing|
By Sweeper tamonten,China Incident Photograph Album, Vol 2, published in 1938 by Asahi Shimbun., Public Domain, accessed at Wikimedia Commons.
We know hardly anything about the war in China.
To give just one example, about 80,000 Chinese and Japanese soldiers became casualties during the first battle for the city of Changsha in September and October 1939 (there were three more battles for the city later in the war.) This is more than twice the number of total casualties on both sides during Operation Market Garden, the disastrous British and American attempt in September 1944 to penetrate German defenses in a bold airborne assault.
Hundreds, perhaps thousands of books have been written about that failed Allied offensive, focusing especially on its tragic epicenter at Arnhem. By contrast, not a single book exists in any western language about the first battle of Changsha – or the second, third and fourth, for that matter.
Comparisons such as these could go on for a very long time. Whereas biographies of US General George Patton are too numerous to count, no books exist in the West about flamboyant commanders such as Chinese Muslim General Bai Chongxi or Sun Liren, described as “China’s Rommel.” How many people in the West know that in China local puppet troops were doing a lot of Japan’s “dirty work”? Or that the first American firebombing in Asia was against the Chinese city of Wuhan in December 1944? 
These type of comparisons will be familiar to anyone who has been reading The Scholar's Stage over the last few years, as I have made this same point again and again (and again!). Harmsen does not find the common explanations for the gap at all convincing:
Various explanations can be put forward to account for this gaping hole in the historiography. First, it can be argued that the China theatre was not decisive in the same way that, for example, the Eastern Front was. Even in a regional context it was arguably a sideshow. The war against Japan was decided on the small islands of the Pacific, not in China’s interior. According to this argument, post-war historians have shied away from this subject simply because it wasn’t important enough.Harmsen misses one explanation I sometimes hear bandied about: in the early years of the Cold War the history of the Sino-Japanese War was a toxic topic. More specifically, Americans could not write about it without declaring an answer to the question "who lost China?", and in the 1950s that was a dangerous question to answer. But this explanation has its own limitations; it does nothing to explain the lack of scholarship done in other parts of the Anglophone world, and it really only accounts for a decade or so of lost time. By the time Frances Fitzgerald published Fire on the Lake in 1972, sympathetic portrayals of America's communist enemies and biting critique of American blunders were workaday projects for American journalists and historians alike.
The argument is not particularly convincing [for lots of reasons readers of the Stage likely know already]....
Another possible explanation for the low level of interest in China’s struggle is the absence of a consensus narrative about the war.....
This argument has been weakened by the recent thaw in relations between China and Taiwan, reflected in growing recognition among Chinese historians of the key role played by Chiang Kai-shek and the forces under his command. However, what really destroys the argument is the fact that, the Cold War notwithstanding, it would have been possible for American and Taiwan historians to collaborate on histories of World War Two in China from a Nationalist perspective right from the 1950s. It just didn’t happen to any major extent.
Finally, the lack of interest in China’s World War Two experience has been blamed on the difficulty of using China’s archives. This is potentially critically important, as reflected in what happened after Soviet collapse and the opening of the Russian archives. The possibility of suddenly telling the Russian side of the story triggered an explosion in the literature about the Eastern Front.
Does this argument have relevance for China? Yes and no. It’s true that Chinese archives may have been out of bounds during the Cold War, but today, serious Western historians have much easier access. In addition, both China and Taiwan have published and continue to publish carefully prepared historical source materials, providing valuable information for anyone able to read them. 
Harmsen thinks that the best explanation is a linguistic one:
This leads to the circumstance which I consider the main obstacle to western research into the war in China: the difficulty of the Chinese language. It is a problem that’s not always recognized, but it’s nonetheless very real. According to the Foreign Service Institute at the State Department, it takes 2,200 class hours of devoted study to achieve proficiency in Chinese. This is about twice the amount of time needed to learn Russian or Vietnamese, and four times as much as the time invested in learning French or Dutch.Harmsen's hypothesis is correct, but it is most compelling when placed in the larger context of Chinese historiography. The best way to approach this context is through comparison. The Foreign Service Institute ranks Arabic at the same level of difficulty as Chinese. How does the historiography of the Middle Eastern military history compare with that of Chinese military history? You do not need to spend much time in a library to realize that academic works on the most famous wars of Middle Eastern history have at least as much written about them by academic historians, and far more written by popular historians, than any conflict China has been a part of. Language difficulty alone is not enough to explain the dearth of Chinese military history.
This is just in order to learn the modern Chinese language. To truly grasp the Second Sino-Japanese War in all its complex intricacy, knowledge of the classical Chinese language is a definite advantage, too. For example, Chiang Kai-shek’s diary, possibly the most important primary source of them all, was written in a terse and elliptical style which comes across as archaic even to many Chinese.
Unfortunately, knowledge of the Chinese language is absolutely crucial in order to do more than just scratch the surface of the complex events in China in the years from 1937 to 1945. Speaking from personal experience, if I hadn’t been able to read Chinese, I could never have completed my own two books on the subject, Nanjing 1937: Battle for a Doomed City and Shanghai 1937: Stalingrad on the Yangtze.
What to do about this situation? The answer is simple: nothing. Just wait. Mandarin proficiency is rapidly catching on throughout the West, as young people prepare for a future in which China will be increasingly important economically, politically and militarily. This will also feed into the historians’ profession, where Mandarin will no longer be such a rare skill as it is today. Based on this, I feel confident that in a decade or two, the bookshops and websites will be brimming with books about China’s epic struggle with Japan in the 1930s and 1940s. (Emphasis added). 
The comparison with Middle Eastern history is revealing in other ways. The Crusades and the Arab conquests are some of the most popular topics in Middle Eastern history. These wars were also waged more than a thousand years ago. There is no Chinese war of comparable antiquity that can claim such popularity with Western readers or writers today. Books on modern Chinese military history are meager when compared to books on modern Western military history. Compare books on China's modern military history with books on its pre-modern military history, however, and a different picture emerges. From this perspective, China's experience in the Second World War has an incredible amount written about it. There are more books in English concerning China's eight year long ordeal in the Second World War than there are concerning every war fought by the Chinese during all four centuries of the Han Dynasty combined.
The problem is not that the world knows so little about China's wartime experience in the Second World War. The problem is that the world knows so little about China's wartime experiences period.
I explored the different reasons why this is the case fairly recently in the post "East Asian Military History - A Few Historiographical Notes," so I will not rehash my entire argument here. I will, however, quote the part most relevant to Harmsen's point about how difficult it is to learn Chinese:
Contemporary historians of East Asia have the same basic set of priorities as the rest of their profession. They focus on structures, cultures, identities, and the hidden voices of history... [but] the idea that East Asianists need to counter the biases of existing, politics-heavy narratives is mistaken, for in too many cases there are no existing narratives to counter in the first place. We are left with huge gaps in the literature. In the case of military history, there are entire wars where millions of people fought and died, and whose stories are instantly recognized by people across China, Japan, and Korea today, that still have no books written about them in English.Academics who specialize in Asia are hardly alone in their decision to shy away from writing about war and conflict. The vast majority of academics with PhDs in American studies don't write histories of World War II either. By and large, the hundreds of WWII books Harmsen references were written by people outside of academia (and many of those with an academic background who write on the Second World War, like Victor Davis Hanson, are specialists in entirely separate eras). They can do this because understanding the primary sources used to write histories of American and British campaigns does not require years of specialized academic training--exactly the short of training most Westerners must get just to speak passable Mandarin Chinese.
Part of the problem is size. The number of East Asianists in academia is small. The number working on pre-modern East Asian history is pitifully small. You can count the number of American scholars who specialize in Silla Korea on one hand. You could count those who specialize in Sengoku Japan on two. You could fit all the specialists on the Northern Song Dynasty on a moderately sized tour bus. This is true now; back when narrative political and military histories was more academically fashionable (c. 1920-1960) the number of East Asianists were even smaller. Because only a few scholars specialized in East Asia then, the peculiar research interests of one scholar and his pupils forty years ago have come to dominate entire fields today (one example of this is substantial amount of work done on medieval Japan's institutional history, something I credit entirely to the influence of John Whitney Hall, who taught Japanese history at Yale for the better part of the last century). There simply weren't enough historians writing then to fill in the gaps.
In addition, many of those who wrote then were relatively unconcerned with high politics, diplomacy, or military affairs. They came to the study of traditional Asia with a set of non-traditional backgrounds. Then—as now—a great deal of East Asian history is written by philosophers, philologists, and archaeologists. These are men and women who began to study East Asia because of a fascination with Pure Land Buddhism, Neoconfucian metaphysics, Shang Dynasty bronzes, or reconstructing classical Chinese pronunciation. In most areas linguistics, philosophy, literature, and religious studies are separate fields, but in the case of the East Asianists (and here a fruitful analogy with the Classicists can be made) they blur somewhat. The very term "East Asianist" (along with its subsets: "Sinologist,""Koreanist," etc.) express the expectation that those studying one aspect of pre-modern Asia should be conversant in all of its other domains. In this milieu intellectual history has always been king. This is partially because many of these disciplines began as an attempt to make the "Eastern mind" accessible to Westerners, and it is partially because it is incredibly difficult to understand even fairly mundane historical sources without a working knowledge of classical Chinese and the history of ideas in East Asia.  The interdisciplinary nature of this sort of intellectual history sheltered it somewhat from the political storms and of the '60s and '70s. It is still the strongest strain of historical scholarship on the region.
The downstream effects of all this are pretty easy to see. By far the most common textbook for introductory survey courses of East Asian history is Sources of the East Asian Tradition, a collection of mostly philosophical and literary documents from the last few thousand years of East Asian history. The Association for Asian Studies annual conference rarely has panels on the political or military history of Sengoku Japan, but there will always be room for one more panel on the Tale of Genji to be squeezed in. 
Boiling this argument down to a few bullet points leaves us with the following:
We know so little about WWII because
1) Academic historians shy away from writing about high politics or warfare
2) The difficulty of the Chinese language keeps the majority of popular historians far away from the topic.Harmsen is proof of the point. He studied history at National Taiwan University in Taipei, but he made his name as a journalist, not a historian. That is probably for the better. Had he continued on to get a PhD in history the urge to write compelling narrative histories--something he is quite good at--may very well have been beaten out of him.
 Peter Harmsen, "Why Do We Know So Little About China in World War Two?", History News Network (December 13, 2015).
[4[ T. Greer, "East Asian Military History - A Few Historiographical Notes," The Scholar's Stage (18 December, 2015).