The South China Sea is the most important body of water for the world economy—through it passes at least one-third of global trade. It is also the most dangerous body of water in the world, the place where the militaries of the United States and China could most easily collide.

Chinese and American warships have just barely averted several incidents there over the past few years, and the Chinese military has warned off U.S. jets flying above. In July, the two nations carried out competing naval exercises in those waters. Given what is called the growing “strategic rivalry” between Washington and Beijing, the specter of an accident that in turn triggers a larger military confrontation preoccupies strategists in both capitals.

These tensions grow out of a disagreement between the two countries as to whether the South China Sea is Chinese territory, a quarrel that speaks to a deeper dispute about maritime sovereignty, how it is decided upon, and the fundamental rights of movement in those waters.

The standoff over the South China Sea thus has many levels of complexity. It is not simply about one body of water, or a single boundary. As Tommy Koh, a senior Singaporean diplomat who led negotiations to create the United Nations Convention on the Law of the Sea, told me, “the South China Sea is about law, power, and resources, and about history.”

That history is haunted in particular by four ghosts, long-departed men from centuries past whose shadows fall across the South China Sea, their legacies shaping the deepening rivalry in the region; historical figures whose lives and work have framed the disputes about sovereignty and freedom of navigation, the competition of navies, as well as war and its costs.

During the writing of my book, The New Map, I began thinking about these men. When I was speaking on the challenges of globalization and international commerce at the U.S. Naval War College in Newport, Rhode Island, the commanders of virtually all the world’s navies were there, a galaxy of admirals, all resplendent in their dress uniforms. Among them was Admiral Wu Shengli, the head of China’s navy at the time and the man who was driving its expansion to compete with the American Navy. By then the South China Sea had already become a center of contention. Wu sat in the center of the audience, in the fifth or sixth row, his gaze unwavering throughout.

That was when I started seeing ghosts: that of China’s greatest seafarer, a predecessor to Wu; of the Dutch lawyer who penned the legal brief that now underpins the American argument against China’s claims; of the American admiral whose philosophies offered a foundation for both the U.S. Navy and Chinese maritime expansionism; and of the British writer who argued that the costs of conflict were too high, even for those who would be victorious.

For modern China, claims to the South China Sea center around what is called the “nine-dash line”—literal dashes that, on the Chinese map, hug the coasts of other nations and encompass 90 percent of the waters of the South China Sea. Derived from a map drawn by a Chinese cartographer in 1936 in response to what Beijing calls the “century of humiliation,” the nine-dash line is, according to Shan Zhiqiang, the former editor of China’s National Geography magazine, “now deeply engraved in the hearts and minds of the Chinese people.” Chinese schoolchildren have for decades been taught that their country’s border extends more than a thousand miles to the coast of Malaysia. Beijing’s claims are bolstered by military bases that it has built in recent years on tiny islands and on 3,200 acres of reclaimed land scattered in the middle of the sea.

Beijing bases its claim of “indisputable sovereignty” upon history—that, as an official position paper put it, “Chinese activities in the South China Sea date back to over two thousand years ago.” These “historic claims,” in the words of a Chinese government think tank, have “a foundation in international law, including the customary law of discovery, occupation, and historic title.”

The U.S. replies that, under international law, the South China Sea is an open water—what is often called “Asia’s maritime commons”—for all nations, a view shared by the countries that border its waters, as well as by Australia, Britain, and Japan. As such, says the U.S. State Department, China “has no legal grounds” for its South China claims and “no coherent legal basis” for the nine-dash line. “China’s maritime claims,” a U.S. government policy paper argued this year, “pose the greatest threat to the freedom of the seas in modern times.”

And this brings us to the four ghosts.

Zheng He’s sailing charts were published in 1628, depicting India and Africa. (Universal History Archive / Getty)

In 1381, during a battle in southwest China, a Muslim boy was captured by soldiers of the Ming dynasty, castrated, and sent to work in the royal household of Prince Zhu Di. As time went on, the boy—renamed Zheng He—grew up to become a confidant of the prince and, eventually, one of his most able military leaders.

When Zhu became emperor, determined that China must be a great maritime power, he ordered a frenetic shipbuilding campaign that launched huge fleets carrying up to 30,000 personnel. They transported both a wide range of Chinese goods and the most advanced ordnance of the day—guns, cannonballs, and rockets. The biggest boats were treasure ships that were as much as 10 times larger in capacity than those Christopher Columbus would captain to the New World almost a century later. These Chinese voyages would take two or three years, with eunuchs in command of each of the fleets. But the commander in chief, above all others, was Zheng. He eventually became known as the Three-Jewel Eunuch, in honor of the “three jewels” central to the dominant Buddhist faith of Zhu’s reign.

Admiral Zheng’s first voyage, in 1405, was put to sea with an armada of more than 250 ships, of which more than 60 were treasure ships. Altogether Zheng commanded seven voyages, some sailing as far as the east coast of Africa, to modern Kenya. Along the way, his fleet would trade Chinese goods and products with the locals, while projecting the power and majesty of China—in Zheng’s words, “making manifest the transforming power of imperial virtue.” One can imagine the impact on those ashore when they caught sight of the approaching giant fleets, and especially the huge treasure ships, with their tail sails filling the skies, their fierce dragon eyes painted on their prows, bearing down on the shore.

Upon their return to China, Zheng’s fleets brought back not only a wide variety of products and novelties—including precious stones, spices, camels, and ostriches—but also rulers and ambassadors, who would pay homage and tribute before the emperor. Zheng’s armadas, as the historian John Keay has written, also “demonstrated maritime mastery of the entire Indian Ocean.”

An illustration depicting Zheng He. (Universal History Archive / Universal Images Group / Getty)