At last.

After weeks of Hamlet-esque public debate, U.S. defense officials ordered Aegis destroyer USS Lassen to cruise within 12 nautical miles (nm) of Subi Reef on Oct. 27.

The reef is an undersea rock in the South China Sea that Beijing has built into an artificial island in the contested Spratly Islands. It wants it to be considered a real island, with a “territorial sea” surrounding it. That means a 12-nm zone where Chinese domestic law prevails, just like Beijing.

China claims it has the right to do so based on “historic” claims to most of the South China Sea, an area through which $5 trillion dollars worth of trade passes.

By sending a warship within that zone, the U.S. Navy signaled that the United States rejects efforts to rewrite the rules governing the sea and sky. International law clearly states the open sea is no one’s property, and such “freedom-of-navigation” voyages are standard fare elsewhere in the seven seas. And the Lassen’s cruise can’t be a one-time trip without giving China another opportunity to assert its unlawful authority.

The nice thing about the law of the sea is that it’s well written. The UN Convention on the Law of the Sea (UNCLOS), says coastal states may construct artificial islands within “exclusive economic zones” extending 200 nautical miles off their coasts. Beyond that limit, the law allows no such projects.

Now fire up Google Earth. Subi Reef lies 500 nautical miles from Hainan Island, the nearest Chinese shoreline. It sits far closer to the Philippines — only 230 nautical miles from the island of Palawan. Manila, then, has a better legal claim than Beijing by sheer geographic proximity — but even Philippine land “reclamation” there would be outside the law.

Subi Reef was a submerged atoll before Chinese engineers dredged up the sea floor to create an island, and then topped it off with an airstrip. This manufactured turf has no legal status — yet Beijing is trying to give it legal status by fiat.

An artificial island that falls within a country’s exclusive economic zone merits a concentric 500-meter safety zone. That’s next to nothing in navigation and piloting terms. If it falls beyond the zone, like Subi Reef, it merits nothing at all — much less a 12-nautical-mile territorial sea. Indeed, Lassen could have ventured as close to Subi Reef as its skipper dared while still remaining within the law.

Critics of Washington’s enforcement of international law often criticize the United States for not being party to UNCLOS. The reality is that the accord would have no force but for the U.S. Navy’s efforts to challenge and overturn excessive claims to land, sea or sky.

But methods matter. Why send the fleet to contest a complicated legal point? Why not appeal to some international court? The fact is, domestic law provides remedies for wrongful claims. The law of the sea does not. Or, more precisely, a UN tribunal exists to resolve nautical disputes, but seagoing states can refuse to accept its authority — as China has done in a case brought by the Philippines.

That leaves seafaring states mindful of maritime freedom with an unpleasant choice. They can either accept the unacceptable, namely China’s effort to rewrite international law to the detriment of maritime freedom. Or they can resort to self-help. Governments can defy unlawful claims by deploying steel, à la Lassen; by backing up their actions through diplomatic correspondence with the offending government; and by explaining their purposes to important audiences. If resolute enough, stakeholders in the free sea keep excessive claims from calcifying into international custom and — perhaps — law.

Such measures matter because of the nature of international law. Lawmaking isn’t all about drawing up solemn accords. What governments do — or refrain from doing — also helps make law. Fail to object to unlawful claims or deeds over time, and you consent. If enough governments appear to consent, the original law eventually loses its force.

Beijing is banking on it.

The cruise of the Lassen, then, was neither novel nor radical. Such operations constitute longstanding, bipartisan U.S. policy toward freedom of the sea. Indeed, Washington’s Freedom of Navigation Program dates to the first Reagan administration. It’s as old as UNCLOS itself.

So, what’s next?

If the United States is serious about keeping sea routes free, it will have to challenge China’s claims by sending ships and planes into embattled waters and skies as a matter of course, not as a one-time show of force.

By contrast, if this week’s demonstration ends up being a one-off gesture, U.S. leaders will have admitted that one government can unilaterally abridge — or conceivably abolish — laws that underpin the system of maritime trade and commerce over which America presides.

Propaganda Battle

But dispatching ships and planes is not enough. Washington must explain its reason for its challenges to unwarranted claims, and it must do so early and often. It must not let Beijing define what the U.S. Navy is doing, as it has done in the past.

Chinese officials have claimed that the United States is “militarizing” South China Sea quarrels such as the one over Subi Reef. Last month, during his state visit to Washington, President Xi Jinping pledged not to “militarize” China’s artificial islands, even though engineers have built airfields long enough to accommodate combat aircraft of all varieties.

However, Subi Reef’s airstrip will remain demilitarized — until the first warbird touches down. Then it’s militarized. China’s artificial islands are demilitarized the same way Naval Station Newport, Rhode Island, is demilitarized. The navy no longer chooses to base warships at the piers in Newport, which is now a training and education facility. But let a man-of-war show up in Narragansett Bay, and presto! Newport reverts to its militarized status.

China is no stranger to muscle-bound island policies. In 1974, for instance, Chinese forces pummeled South Vietnamese forces in the Paracels — also in the South China Sea — wresting islands from Saigon.

And the pattern has held. In 2001, a Chinese fighter pilot hot-dogging near a U.S. Navy surveillance plane caused a midair collision.

More recently, the cruiser USS Cowpens was harassed in 2013. A ship from China’s navy was responsible. And in 2014, a Chinese jet “jumped” a U.S. Navy P-8 Poseidon surveillance plane flying near Hainan. The Chinese J-11 came within 20 feet of the U.S. aircraft.

Beijing, in short, has been quick to reach for the military instrument over the years. Dispatching vessels to combat Chinese maritime claims is necessary for success in Southeast Asia, but it’s not sufficient. American military spokesmen must arm themselves with basic facts about law and history if they’re to fight — and win — the war of words that’s surely coming.

There’s an even more basic point: navies enforce the law of the sea, and always have. Who else will do it — the Maersk Line? A Carnival cruise?