According to a United States defense official, an American naval destroyer cruised within just 12 nautical miles of a Chinese manmade island in the South China Sea early Tuesday challenging the eastern superpower’s territorial claims.

The guided-missile destroyer, the USS Lassen, wasn’t alone either. It was accompanied by naval surveillance aircraft near the Subi Reef in the Spratly Islands. The official who leaked the intelligence did so anonymously, and added that the mission was “completed without incident”.

That is, yet.

The action goes against China’s directive in the region, which was solidified last month when they said that they would “never allow any country” to trespass their territorial waters and airspace surrounding the island.

Sources report that, currently, Chinese defense officials were still in the process of confirming whether or not the destroyer had penetrated the 12-mile zone.

They even released this loaded language concerning the incident, from the mouth of their foreign minister Wang Yi:

“If true, we advise the U.S. to think again, not to act blindly or make trouble out of nothing.”

More from the Washington Post:

China claims almost all of the South China Sea as its territory, including the main islands and reefs, and has argued that giving up that claim would “shame its ancestors.” The Philippines, Vietnam, Malaysia, Brunei and Taiwan also have overlapping claims, and several of them also occupy different islands, reefs and rocks.

A massive Chinese program of land reclamation and construction on several islands has taken place since 2014, upsetting ties with the United States and several of those rival claimants.

This week’s U.S. naval mission is also intended to test a pledge made by President Xi Jinping during his visit to Washington last month that Beijing would not militarize the islands, U.S. officials have said.

Right now, it certainly seems like Washington called China’s bluff. The Subi Reef, which is where the island today stands, used to be nothing more than a sometimes-exposed rock in the middle of the sea. A recent (and very expensive) dredging project by the People’s Republic turned it into a full-fledge island, however, and according to U.S. officials, it’s not big enough to house a military airstrip.

While the airstrip doesn’t exist on the isle yet (one is under construction elsewhere, however), satellite antennas and a surveillance tower do.

At the heart of this destroyer sailing expedition is a much larger point; the U.S. is trying to reinforce the international law of the sea, which clearly states that turning a reef into a makeshift island in no way designates rights to the waters around it. Rather, it offers said nation a 500-foot “safety zone” field, and nothing more.

The question now is this: who backs down? The U.S.? Or China?