A mere 25 miles off the shore of Palawan sits the frontier of an increasingly dangerous and unpredictable struggle. Its origin lies in China’s intensifying efforts to remake the maritime borders of this region, just as surely as Russia is remaking Europe’s political map in places like Crimea and Ukraine—only here the scale is vastly larger, the players more numerous, and the complexity greater.

Moving with ever greater boldness, Beijing has begun pressing claims to ownership of more than 80 percent of the South China Sea, waters enclosed by what it calls its “nine-dash line,” a relic of the country’s early-20th-century nationalist era, when it was first sketched to indicate China’s view of its traditional prerogatives. The line has no international standing and had gone largely unremarked upon until China recently revived it. It now figures in all Chinese maps. Since 2012, it has been embossed in new passports issued to the country’s citizens.

Also known as the cow’s tongue, for the way it dangles from China’s southern coast, the line encloses a region through which roughly 40 percent of the world’s trade and a great majority of China’s imported oil passes, via the Strait of Malacca, as through the eye of a needle. An observation from the 16th century—“Whoever is lord of Malacca has his hand on the throat of Venice”—still conveys the region’s maritime importance.

Residents of outposts like Palawan, which sits along the eastern edge of the nine-dash line, already feel besieged. Fishermen who enter waters that their forebears freely traversed for generations nowadays find themselves at risk in a disputed no-man’s-land. “The locals are afraid to go out to the west because there are a lot of Chinese boats—military vessels,” said Edwin Seracarpio, a 52-year-old boat owner whom I found one bright morning waiting port-side for the return of one of his crews. “The Chinese say it has always been their property.”

If China can impose its will in the South China Sea, at least five rival claimants—all much smaller, weaker Asian states—will be limited to a narrow band of the sea along their coastlines. China would gain greater security for its crucial supply lines of oil and other commodities; exclusive access to rich fishing areas and potentially vast undersea oil deposits; a much larger buffer against what it regards as U.S. naval intrusions; and, not least, the prestige and standing it has long sought, becoming in effect the Pacific’s hegemon, and positioning itself to press its decades-old demand that Taiwan come under its control. Arguably, it would achieve the greatest territorial expansion by any power since imperial Japan’s annexation of large swaths of Asia in the first half of the 20th century.

China’s expansion has long been expected. Many observers have said a new cold war, in which a rising China gradually seeks to push the U.S. military out of the western Pacific, is inevitable. Any such conflict would of course be dangerous whenever it happened, because the United States is likely to resist these efforts strenuously. But what’s surprising—and worrisome—is how the timeline for this conflict, or at least its beginning stage, has seemed to accelerate over roughly the past two years. Suddenly and aggressively, China has begun advancing its military interests throughout the region, catching its neighbors and the United States off guard.