A potentially dangerous showdown is quickly building between China and not just its neighbors but the United States over freedom of navigation in the South China Sea, one of the world’s busiest waterways.

The Obama administration should exercise firmness against this challenge to international order. But it also should galvanize a reasoned international response that lowers the threat of armed conflict.

Though that’s a delicate balance, it’s a critical task necessitated by China’s unacceptable behavior.

In a bizarre, blatantly unlawful land grab designed to assert control over the entire South China Sea, China has been building artificial islands on coral reefs in the Spratlys, an oil-bearing chain of reefs 800 miles from the Chinese mainland and within the economic zones of, and claimed by, the Philippines, Vietnam and Malaysia.

There has been a predictable outcry of protest from those countries and mounting concern in Tokyo and Washington that freedom of navigation of the South China Sea could be at risk if China persists in those construction projects and outlandish claims. And on Monday, the Philippines’ defense minister said his nation is seeking a “stronger commitment” from the U.S. to stop China’s bogus claims of sovereignty over those islands.

The Pentagon did recently announce that it is considering ways to assert freedom of navigation in the contested waters with overflights and visits by naval vessels.

China immediately denounced the plans. While asserting that China supports “freedom of navigation” the Chinese foreign ministry said “freedom of navigation definitely does not mean the military vessel or aircraft of a foreign country can willfully enter the territorial waters or airspace of another country.”

This statement appears to be a Chinese contention of right to control all passage through the South China Sea. That is obviously not “freedom of navigation” as understood in maritime law for ocean zones beyond the 12-mile limit.

Last week, CNN reported that the Chinese navy issued warnings eight times as a U.S. surveillance plane flew over the man-made islands in the Spratlys that Beijing is using to extend its reach. CIA Deputy Director Michael Morell told CNN there is “absolutely” a risk of war in a potential naval confrontation.

But if its claim goes uncontested China will soon control a major sea lane. The U.S. and other concerned nations should continue to mount a deliberate but low-profile show of force with military vessels and aircraft to defend their right to navigate the South China Sea and its airspace without hindrance.

A show of force that is multinational, unified and continuous would place the burden on China to back down or escalate its assertion of authority over the South China Sea in defiance of nations that are among its most important commercial partners. The looming Trans Pacific Partnership trade deal offers fresh evidence of the high economic stakes for China in maintaining a stable relationship with, among other nations, the U.S. and Japan.

Perhaps Chinese leaders suspect that the Obama administration lacks the stomach to defend international navigation rights that directly affect the U.S. economy.

If so, President Obama must swiftly disabuse them of that assumption by rallying international support that will minimize the risk of a military incident.