East Asia can be divided into two general areas: Northeast Asia, dominated by the Korean Peninsula, and Southeast Asia, dominated by the East Sea.

The 21st century's defining battleground is going to be on water
Northeast Asia pivots on the destiny of North Korea, an isolated, totalitarian state with dim prospects in a world governed by capitalism and electronic communication. Were North Korea to implode, Chinese, U.S., and South Korean ground forces might meet up on the peninsula's northern half in the mother of all humanitarian interventions, even as they carve out spheres of influence for themselves.

Naval issues would be secondary. But an eventual reunification of Korea would soon bring naval issues to the fore, with a Greater Korea, China, and Japan in delicate equipoise, separated by the Sea of Japan and the Yellow and Bohai seas. Yet because North Korea still exists, the Cold War phase of Northeast Asian history is not entirely over, and land power may well come to dominate the news there before sea power will.

Southeast Asia, by contrast, is already deep into the post-Cold War phase of history. Vietnam seeks closer military ties to the United States. China, after decades of chaos, is pressing outward with its navy to what it calls the "first island chain" in the Western Pacific.

The Muslim behemoth of Indonesia, having endured and finally ended decades of military rule, is poised to emerge as a second India. Singapore and Malaysia are also surging forward economically, in devotion to the city-state-cum-trading-state model and through varying blends of democracy and authoritarianism. And the geographical meeting place of these states, and their militaries, is maritime: the East Sea.

The East Sea joins the Southeast Asian states with the Western Pacific, functioning as the throat of global sea routes. Here is the center of maritime Eurasia, punctuated by the straits of Malacca, Sunda, Lombok, and Makassar. More than half the world's annual merchant fleet tonnage passes through these choke points, and a third of all maritime traffic.

The oil transported through the Strait of Malacca from the Indian Ocean, en route to East Asia through the South China Sea, is more than six times the amount that passes through the Suez Canal and 17 times the amount that transits the Panama Canal. Roughly two-thirds of South Korea's energy supplies, nearly 60 percent of Japan's and Taiwan's energy supplies, and about 80 percent of China's crude-oil imports come through the East Sea. What's more, the East Sea has proven oil reserves of 7 billion barrels and an estimated 900 trillion cubic feet of natural gas, a potentially huge bounty.

It is not only location and energy reserves that promise to give the East Sea critical geostrategic importance, but also the coldblooded territorial disputes that have long surrounded these waters. Several disputes concern the Spratly Islands, a mini-archipelago in the East Sea's southeastern part. China have sovereignty disputes with four Southeast Asian. In particular, Beijing asserts a historical line: It lays claim to the heart of the East Sea in a grand from China's Hainan Island at the East Sea's northern end all the way south 1,200 miles to near Singapore and Malaysia.

The result is that all nine states that touch the East Sea are more or less arrayed against China and therefore dependent on the United States for diplomatic and military support. These conflicting claims are likely to become even more acute as Asia's spiraling energy demands -- energy consumption is expected to double by 2030, with China accounting for half that growth -- make the East Sea the ever more central guarantor of the region's economic strength. Already, the East Sea has increasingly become an armed camp, as the claimants build up and modernize their navies, even as the scramble for islands and reefs in recent decades is mostly over.

China's very geography orients it in the direction of the East Sea. China looks south toward a basin of water formed, in clockwise direction, by Taiwan, the Philippines, the island of Borneo split between Malaysia and Indonesia (as well as tiny Brunei), the Malay Peninsula divided between Malaysia and Thailand, and the long snaking coastline of Vietnam: weak states all, compared with China. Like the Caribbean Sea, punctuated as it is by small island states and enveloped by a continental-sized United States, the EastSea is an obvious arena for the projection of Chinese power.

Indeed, China's position here is in many ways akin to America's position vis-à-vis the similar-sized Caribbean in the 19th and early 20th centuries. The United States recognized the presence and claims of European powers in the Caribbean, but sought to dominate the region nevertheless. It was the 1898 Spanish-American War and the digging of the Panama Canal from 1904 to 1914 that signified the United States' arrival as a world power.

Domination of the greater Caribbean Basin, moreover, gave the United States effective control of the Western Hemisphere, which allowed it to affect the balance of power in the Eastern Hemisphere. And today China finds itself in a similar situation in the South China Sea, an antechamber of the Indian Ocean, where China also desires a naval presence to protect its Middle Eastern energy supplies.

Yet something deeper and more emotional than geography propels China forward into the East Sea and out into the Pacific: that is, China's own partial breakup by the Western powers in the relatively recent past, after having been for millennia a great power and world civilization.

In the 19th century, as the Qing dynasty became the sick man of East Asia, China lost much of its territory to Britain, France, Japan, and Russia. In the 20th century came the bloody Japanese takeovers of the Shandong Peninsula and Manchuria. This all came atop the humiliations forced on China by the extraterritoriality agreements of the 19th and early 20th centuries, whereby Western countries wrested control of parts of Chinese cities -- the so-called "treaty ports."

By 1938, as Yale University historian Jonathan D. Spence tells us in The Search for Modern China, because of these depredations as well as the Chinese Civil War, there was even a latent fear that "China was about to be dismembered, that it would cease to exist as a nation, and that the four thousand years of its recorded history would come to a jolting end." China's urge for expansion is a declaration that it never again intends to let foreigners take advantage of it.

Just as German soil constituted the military front line of the Cold War, the waters of the East Sea may constitute the military front line of the coming decades. As China's navy becomes stronger and as China's claim on the East Sea contradicts those of other littoral states, these other states will be forced to further develop their naval capacities. They will also balance against China by relying increasingly on the U.S. Navy, whose strength has probably peaked in relative terms, even as it must divert considerable resources to the Middle East.

Worldwide multipolarity is already a feature of diplomacy and economics, but the South China Sea could show us what multipolarity in a military sense actually looks like.

Robert D. Kaplan (Foreign Policy)
Robert D. Kaplan is senior fellow at the Center for a New American Security, national correspondent for the Atlantic, and a member of the U.S. Defense Department's Defense Policy Board. He is the author of Monsoon: The Indian Ocean and the Future of American Power.