Who was Sun Tzu writing for? Given that Art of War gives strategic principles in what are necessarily adversarial circumstances, how did it become widely circulated? Would that not have been disadvantageous to those who inherited his advice?

This is a very interesting question. In an environment of fiercely hostile states competing for supremacy, it doesn't really make sense for a successful general to write down his knowledge for all to read. And any ruler who got hold of such a document would find it in his best interests to surpress it. So how did a text on military methods end up as one of the most famous and widely read texts from Ancient China?

In many ways, the 孫子兵法 (Sun Zi Bing Fa, "Master Sun's Art of War") is not the most promising place to look for an answer. The origins of the work are shrouded in mystery; the author may or may not be historical; the manuscript tradition is fraught with difficulties; the text itself was summed up by Robin Yates as "terse and obscure" and by Victor Mair as "extraordinarily terse and maddeningly obscure". We'll have to dig into these issues a bit if we are to get any sense of what the Sun Zi was meant to achieve.

First, the origins. At first glance, this question seems easy to answer: in his 史記 (Shi Ji, "Records of the Grand Historian"), the Han Dynasty scholar Sima Qian tells us that the Sun Zi was written around the early 5th century BC by a famous general called Sun Wu. Sima Qian offers some delightful biographical details of this Sun Wu, including the famous story of how he trained the concubines of the king of Wu in heavy infantry drill. The notion that there was indeed a historical Master Sun, who wrote his treatise on warfare at the end of the Spring and Autumn Period, is still held by many. If we adopt this view, the answer to your question is relatively simple: a fugitive from the northern state of Qi (according to one version of the story), Sun Wu needed to impress his new master the king of Wu, and apart from his display as a drillmaster, the compilation of his Thirteen Chapters (the Sun Zi) would have provided evidence of his abilities. The biographical tradition helpfully notes that his advice and leadership allowed Wu to expand and conquer its neighbour, Chu.

The problem with this account of the genesis of the Sun Zi is that there is no earlier account of the life of a general named Sun Wu, and that the very practice of assigning authors to works is not something the Chinese actually did until after the end of the Spring and Autumn period. The find of a collection of texts on strips of bamboo - the traditional recording method - in a group of graves dating to the late 2nd century BC has confirmed that the Sun Zi existed more or less in its currently known form at that time, but there is no evidence that would support a date as early as the 400s BC. Moreover, if it did date that far back, it would be by far the earliest Chinese military treatise, with a gap of well over a century between it and its oldest successor.

Indeed, the text itself has been shown to reflect the military practice of the Warring States period (c. 475-221 BC), not the Spring and Autumn period (c. 771-476 BC) in which it was supposedly written. The well-organised, well-drilled infantry armies envisioned by Master Sun do not belong to the era of chariot-borne warlords and their loose entourages that was the Spring and Autumn period. Its references to crossbows must place it in the period after these weapons were invented (sometime in the 4th century BC). In other words, the text is far more likely to be a product of the 4th century BC than the early 5th; as far as we can tell, the spread and recognition of the work as a military classic belongs to the early Han Dynasty.

It is not surprising, then, that Chinese scholars as early as the 13th century AD have wondered whether the legendary author of the Sun Zi ever really existed. Many modern experts have been persuaded by Jens Østergard Petersen's argument that the details of Sima Qian's biography of Sun Wu are generic, and that all its elements can be found in biographies of other generals too. Sun Wu's name (effectively meaning something like "wandering warrior") is considered too convenient, and his historical context too vague, to take him seriously as a character of the late Spring and Autumn period. In fact, there is robust evidence of the existence of a general and military expert with the surname Sun - but his first name was Bin, and he lived well over a century after the alleged time in which Sun Wu flourished, and he is credited with writing an entirely separate treatise on military matters, which was rediscovered in the same collection that gave us the earliest extant traces of the Sun Zi. It seems all too likely that the family name Sun, famously associated with military expertise, became attached to a growing collection of martial wisdom compiled over the course of the Warring States period. The opening of each chapter of the text with the formula "Master Sun said:" is paradoxically an indication that there was never a "Master Sun" - only a succession of authors legitimising their wisdom through his reputation. Predictably, Sun Bin was eventually credited with being a descendant of the legendary Sun Wu, whose wisdom clearly ran in the family.

The fact is that the way of recording Ancient Chinese texts, using the strips of bamboo mentioned above, is extremely conducive to this sort of textual tradition. Single strips of bamboo contain no more than one or a few lines of text; the strips are held together with string. As the string breaks or decays, texts become jumbled, with later editors often totally unable to restore them to their original order. From our point of view, it is shockingly easy for lines to be added or lost. One of the frustrating things about the Sun Zi is that much of its metatext - its "next"s and "therefore"s - don't actually make any sense, and that not enough serious philological study has been devoted to restoring the text. Given its many uncertainties, we are on far safer ground to assume that the work is the product of many hands over a long period (roughly the 4th and early 3rd centuries BC) than that it is the work of a single, legendary military genius.

So where does that leave us with regard to your question? Why was the Sun Zi written? Who benefitted from writing it and spreading it around?

Here we find a useful parallel in the tradition of Ancient Greek literature. In Greece, as in China, military treatises did not develop in a vaccuum, when a particular military mastermind decided to commit his wisdom to the ages. Rather, they began to appear in an era that saw a general shift in notions of learning, in which oral tradition and abstract thought began to gave way to practical written works that allowed knowledge in discreet fields of expertise to be replicated and taught. In Greece, as in China, the art of war was swept up in this movement by the conditions of the age: in Greece, the hegemonic wars of the Classical period (of which the Peloponnesian War seems to have been especially important as a driving force), and in China, the prolonged and devastating Warring States period. In both cultures, the earliest written works of philosophy had appeared some time before, and kickstarted a tradition of recording advanced thought for the benefit of others. It is not surprising to find Chinese catalogues grouping the Sun Zi in the Daoist (that is, philosophical) tradition. It and the many other military treatises that appeared soon after all fit into the same new approach to the spread of knowledge. The purpose of the text was to teach, so that future generations facing military problems would not have to reinvent the wheel.

Sources

R.D.S. Yates, 'New Light on Ancient Chinese Military Texts: Notes on Their Nature and Evolution, and the Development of Military Specialization in Warring States China', T'oung Pao 74.4/5 (1988), 211-248
J.Ø. Petersen, 'What's in a name? On the sources concerning Sun Wu', Asia Major 5.1 (1992), 1-31
V.H. Mair, The Art of War: Sun Zi's Military Methods (2007)