I’ve long wondered why the Yijing (the Classic/Book of Changes) is so important for ultra-nationalist ideas in Vietnam. Extreme nationalists in Vietnam today regard the Yijing and its ideas as “Việt” and see it as central to Việt culture.
If, however, you look for evidence for this view prior to the twentieth century, you won’t find any. Yes, prior to the twentieth century there were people in the area that we now call Vietnam who knew about the Yijing. There were editions of it published there. There were people who used it for divination, etc.
However, it was not seen as being of core importance. So how did it become more important?
While I don’t have the complete answer to this question, I now at least can identify one development that was important for this transformation. And to understand this development, we need to look at the work of a Chinese intellectual by the name of Hu Shi 胡適.

Hu Shi was one of the most important and influential Chinese intellectuals of the twentieth century. The Wikipedia page about him makes this clear.
For the purposes of our discussion here, however, we need to look at the dissertation that he wrote as a student at Columbia University in the 1910s entitled “The Development of the Logical Method in Ancient China.”
This dissertation was not simply a study of philosophy. Instead, we can also see it as Hu Shi’s attempt to “save” China. At the time he researched and wrote this work, Chinese society was struggling to adapt to a world which now included a powerful alien culture (“Western culture”) that seemed to be radically different from the culture of traditional China.

What is more, that alien culture was gradually extending its dominance into the Chinese world.
Something had to be done, and Hu Shi’s solution was to demonstrate through his dissertation that there actually wasn’t a big difference between Chinese and the Western culture. Instead, some of the attributes that supposedly defined the West, like logical thought, could be found in Chinese culture, and more specifically, in its ancient intellectual tradition.
The problem, according to Hu Shi, was that later intellectual developments (namely the development of Neo-Confucian ideas during the Song-Ming Dynasty period) led Chinese thought into a less-logical direction.
Therefore, Hu Shi sought to push Neo-Confucian ideas to the side and shine a light on the logical ideas that could be found in ancient Chinese thought so that modern Chinese could have the confidence and strength to deal with “the threat of the West.”

What is an example of logical thought in ancient China? Well, Hu Shi pointed to texts like the Yijing. Here is what he said:
“It has been said that the Platonic logic originated as a reaction against the Heraclitean doctrine of change; that, impressed by the all-pervasiveness of change, Plato sought and found stability in the changeless “ideas” [or Forms]. It is significant that the book which, in my opinion, contains most of the basic doctrines of the Confucian logic is known as the Yi, or Book of Change[s].” (pg. 28)
“That the great complexity of change can be symbolically represented by a set of figures which in turn can further be reduced to the elemental line (-), is a fact which seems to have deeply impressed Confucius in the same way as numbers impressed the Pythagoreans and Platonists. Herein is found a perfect system by means of which all change in the universe can be brought under our examination and understanding.” (pg. 32).

In other words, the Yijing was important to Hu Shi because it showed that you could find in Chinese antiquity some of the same ideas that existed at that time in the West, and which were seen to have laid the foundation for the West’s culture.
China was therefore not inferior to the West. Thus, there was hope for the future.

Now let us move forward in time about a decade to the year 1929 when Vietnamese intellectual Trần Trọng Kim published the first volume of his two-volume work, Confucianism (Nho giáo).
Like Hu Shi, Trần Trọng Kim felt that Confucianism had gone off track. In the case of Vietnam, Trần Trọng Kim argued that the civil service examination had been particularly harmful in diverting people’s attention towards learning how to write beautiful but superficial writings in order to pass the exams rather than focusing on the ideas of the Confucian tradition.
Also like Hu Shi, Trần Trọng Kim pointed to parallels between ancient Greek thought and ideas in the Yijing.

Trần Trọng Kim notes, for instance, that around the same time that Confucius was active, in the Greek world there were people like Heraclitus who argued that the all things in the world are in a constant state of change, and Parmenides who held that what people perceive are illusions and that the world is a single unified substance, and Pythagoras who used arithmetic to establish the theory of Pantheism (the idea God and the laws of the universe are the same) and argued that there is a mathematical logic to the unity of the world, and Socrates whose philosophy of life emphasized human affairs rather than higher powers.
He then states that, “Based on [the principles] of Yijing studies, Confucianism uses broken and solid lines to reveal the changes of the law of nature, and uses even and odd numbers to calculate the evolving destiny of the world, which is similar to the arithmetic of the Pythagoreans.
“However, there is one slight difference, and that is that Confucianism holds that although Heaven produces the myriad things, each living object has the freedom to act on its own, and in accordance with the law of nature, after it dies, it seems that its spirit maintains its unique character and continues to circulate and transform, and does not completely meld with the Universe (Đại toàn thể) as living things do according to the Pantheists in the West.”

Hu Shi and Trần Trọng Kim both demonstrated that the Yijing had ideas that were similar to ideas of certain ancient Greek philosophers for the same reason. They were both living in a time when their own cultural tradition was under threat, and they both sought to deal with this situation by showing their readers that their own cultural tradition was not as weak as people actually feared.
As it turns out, the Yijing was the text that they both found they could use to make this argument (Hu Shi also talks about the Laozi, but that’s another story).
Later in the twentieth century, South Vietnamese philosopher Lương Kim Định also turned to the Yijing for similar reasons. And at the end of the twentieth century the Yijing became central to an ultra-nationalist textbook that Trần Ngọc Thêm wrote.
Each of these individuals, from Hu Shi to Trần Ngọc Thêm, wrote under the shadow of a fear of the West’s power and influence, and a fear of their own culture’s inability to adequately adapt to a Westernizing world.
They also all turned to the same text for help, the Yijing, as this was the one text that they felt could show that “their culture” was equivalent to the West’s.
The irony of course is that the need to say that one’s culture is not inferior is already an indication that a writer fears that it is. . . but again, that is yet another story.