At the moment, the war is just one of words. On Friday, the Chinese Foreign Ministry called a U.S. Navy reconnaissance flight over the hotly contested South China Sea “very irresponsible and dangerous and detrimental to regional peace and stability.” The admonition came two days after the Chinese navy sent eight warnings to an American P-8A Poseidon patrol aircraft, telling it not to approach Fiery Cross Reef in the Spratly Island chain.

Beijing and Washington in recent months have been making many declarations about—and trading accusations and warnings over—these 3.5 million square kilometers of water that are roughly bounded by Hong Kong, Taiwan, the Philippines, Indonesia, and Vietnam, where annual commerce totals $5.3 trillion. About half the world’s oil-tanker shipments transit its waters. Six of the world’s 10 busiest ports dot its coasts.

“The South China Sea is the cockpit of geopolitics in East Asia,” wrote the International Crisis Group this month. Yet it may soon be more than just that. This body of water is where history’s next major armed conflict could very well start.

Beijing claims as sovereign territory the largely uninhabited Spratly Islands and virtually all the other atolls, shoals, rocks, and reefs in the South China Sea. Official Chinese maps contain either nine or 10 “dashes” forming “the cow’s tongue” of its self-declared boundary, covering some four-fifths of South China Sea water. The tongue hugs the coastlines of Taiwan and five other countries and extends about 1,800 kilometers from China’s closest shore.

China has not fully clarified the precise extent of its sovereignty claims to South China Sea waters, but it is nonetheless clear that those claims are inconsistent with its treaty obligations and international law. Accordingly, the issuance of the maps is seen as an attempt to exclude other nations from the area.

Taiwan formally maintains the same expansive claims as China but does not seek to enforce them, except on scattered islands. The Philippines, Brunei, Malaysia, Vietnam, and Indonesia maintain competing claims to the islands and other features in the sea close to their shores.

Some countries have engaged in miscellaneous bits of dredging and construction on features they occupy, but none of them has come close to Beijing’s accelerated land “reclamation” program in the Spratlys, where there was once little more than a collection of coral reefs and, in fact, little land.

Since the middle of last year, China has, by dredging reefs and shoals, added approximately 4 square kilometers of land to the Spratlys, including Fiery Cross Reef, which is much closer to Indonesia, the Philippines, Malaysia, Vietnam, and oil-rich Brunei than it is to China. There are, at this time, no fewer than 100 Chinese dredgers at work in the Spratlys.

“The speed, scale, intensity, and remoteness of China’s ongoing manufacture of land and infrastructure within the South China Sea have few or no parallels in history outside of wartime,” writes Victor Robert Lee, a reporter blogging for the Diplomat website. Adm. Harry Harris, commander of the U.S. Pacific Fleet, in March labeled China’s “unprecedented land reclamation” the “great wall of sand.”

Beijing has been cagey about its intended uses of the reclaimed land, but it is apparent the People’s Liberation Army will be a major part of its plans in the Spratlys. The runway on Fiery Cross Reef, for instance, is extremely long, about 3,000 meters.

Specifically, there are worries that China’s new facilities will be used to enforce a South China Sea air-defense identification zone. Such a zone, complementing its East China Sea zone declared in November 2013, would represent a large expansion of Beijing’s reach.

Even without a formal declaration of an air zone over the South China Sea, it appears Beijing already has imposed one. Last Wednesday, the radio warning from the Chinese navy to the American P-8A specifically referred to “our military alert zone.” After the incident, Hong Lei, a spokesman for China’s Foreign Ministry, obliquely referred to the new zone. “The Chinese side is entitled to monitor the situation in relevant waters and airspace,” he said at a regular briefing.

Moreover, Beijing claims that the reclaimed features give rise to a 12-nautical-mile band of territorial water. The claim is inconsistent with international law and Beijing’s obligations under the U.N. Convention on the Law of the Sea, which it has ratified.

The U.S. has no territorial claims in the South China Sea, and because of that Beijing has consistently taken the position that the United States has no legitimate interest there.

Washington disagrees. If there has been any constant in American foreign policy since the birth of the republic, it has been the defense of freedom of navigation. China’s new military alert zone and its claims to territorial waters around reclaimed features certainly infringe on that notion.

And so does China’s attempted jamming of America’s Global Hawk, the Air Force’s long-range drone, near the Spratly chain. The Free Beacon website reported that U.S. officials have mentioned there has been at least one incident of drone jamming.

China’s challenge to the United States in the South China Sea sets up the classic zero-sum confrontation. Beijing has declared that its South China Sea claims are a “core interest” that cannot be negotiated. Washington, which has plied the seas from its very first days as a nation, cannot compromise its defense of the global commons. Each side can make tactical retreats, but neither can abandon its position for long.

There are two competing visions of the world, and only one can prevail. So, going forward, relations between China and the United States will be fundamentally different because of their disagreement over the South China Sea.

No one thinks the Chinese will drop their outlandish claims around the reclaimed features, so all eyes will be on Washington. The U.S., if it is to defend freedom of navigation, will have to fly planes into what Beijing considers its sovereign airspace and drive warships through waters China claims as territorial. “That would be the next step,” Pentagon spokesman Colonel Steve Warren told reporters Thursday.

Senior Colonel Zhou Bo of China’s Ministry of National Defense says China does not want a confrontation, but his country keeps trying to close off international water and airspace to everyone else.

The resulting contest could just be the one that determines whose vision of the world—and which nation’s rules—govern this century.