The showdown over the South China Sea began Tuesday when an international court in The Hague declared that China’s claims to 90% of the world’s critical trade route are bogus. Just like an old-time Western, the lesser-armed folks in town—in this case including Brunei, Malaysia, the Philippines and Vietnam—have scurried off into the buildings lining Main Street. They’ve closed the shutters, leaving them open just enough to peek nervously as China and the United States prepare for a confrontation.

“China’s sweeping, yet undefined, South China Sea claims don’t hold water,” U.S. Naval War College Chinese expert Andrew Erickson said shortly after the ruling. “Looking forward, all parties concerned must prevent China from grabbing with coercion or force what it could not—and now clearly cannot—obtain legally.” The five-member panel unanimously ruled that Beijing’s claim to nearly all of the South China Sea because of its historic presence in the region has no merit. As expected, China quickly rejected the ruling.

So what happens now? The South China Sea has instantly become uncharted waters for the globe’s two most-powerful nations. The ruling from the Netherlands, while legally binding, has no mechanism for enforcement. That means negotiations will be required to ease the growing territorial tensions in and around the South China Sea. If talks don’t happen, or go nowhere—and China continues to refuse to back down—a military clash could occur.

U.S. optimists hope that after an initial outburst, the Chinese will realize the international community has taken a firm stance against its claims in the South China Sea, through which $5 trillion in trade passes annually. Starting bilateral talks on fishing and oil rights between China and the Philippines, which brought the case to The Hague, could ease tensions. The ruling may compel Beijing to curb its dredging in the South China Sea to create new islets claimed as Chinese territory.

But U.S. pessimists suggest China is more likely to increase its island-building, and perhaps impose a blockade on Philippine sailors on a desolate shoal who are based there seeking to declare it as part of the Philippines. The Chinese blockaded the shoal in 2014. “Eventually, Manila ran that blockade by taking a civilian ship, stuffing it full of supplies, stuffing it full of foreign journalists and forcing a difficult decision on the Chinese: `Shoot us out of the water or let us go!’—and the Chinese backed off,” Gregory Poling of the Center for Strategic and International Studies said last month at a CSIS session. “We could very well see a repeat of that episode with a stronger Chinese response.”

That, in turn, could lead to a stronger U.S. response, given the 1951 mutual defense pact between Manila and Washington. A renewed Chinese blockade of the Second Thomas Shoal “would carry with it the highest risk of kinetic interaction”—guns being fired, missiles being launched and/or bombs being dropped—“with the U.S. Air Force or Navy,” Asian expert Michael Green said at the CSIS gathering.

China may also respond by declaring an air-defense identification zone over the South China Sea, as it did over the East China Sea in 2013. That would require foreign flights to identify themselves to China before entering. While civilian airlines have complied, the U.S. military has not. China has been building airstrips on islets in the South China Sea, and could deploy fighter aircraft to them shortly before declaring the sea a second air-defense identification zone. “At the end of the day that’s an elegant target set for the U.S.,” CSIS’s Andrew Shearer said.

A pair of U.S. aircraft carriers is now steaming in the western Pacific. “We don’t get to do two-carrier operations very often,” Admiral John Richardson, the U.S. chief of naval operations, said last month. “It’s a terrific opportunity for us to do some [training for] high-end war-fighting.” But U.S. warships sailing in or near the South China Sea also represent a fat target for China. Beijing has spent years developing and deploying the DF-21D missile, informally known inside the Pentagon as the “carrier killer.”

The decision also is likely to embolden the U.S. to continue—and perhaps step up—its naval patrols in the South China Sea. The U.S. has refused to acknowledge China’s claim of sovereignty to much of it, and repeated “freedom of navigation” exercises through the disputed waters will serve to emphasize the American position, which is widely shared by the non-Chinese nations bordering the sea.

Eyeing one another down that deserted Main Street, both China and the U.S. are wondering if the other is going to reach for its gun. “We have reached a critical turning point,” says Jerry Hendrix, a retired Navy captain now at the Center for a New American Security. “The U.S. and its Navy, in particular, must now consider plans on how it can best support the international community and uphold the rule of law. All options must be on the table.”

Get our Politics Newsletter. The headlines out of Washington never seem to slow. Subscribe to The D.C. Brief to make sense of what matters most. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Contact us at letters@time.com.