American officials told Reuters that the USS Decatur sailed near to Triton and Woody Islands in the Paracels, but did not enter the 12-nautical-mile territorial limit in either case. White House spokesman Josh Earnest declared that the naval operation had been to demonstrate that “coastal states may not unlawfully restrict the navigation rights, freedoms and lawful uses of the sea” by the United States and all other states.

The naval intrusion into disputed waters is the first since July, when the Permanent Court of Arbitration (PCA) in The Hague ruled in favour of a US-backed legal case contesting Chinese claims. The US navy had carried out three previous “freedom of navigation” operations close to Chinese controlled islets, sending a destroyer within 12 nautical miles of Subi Reef in October 2015, Triton Island in January this year and Fiery Cross Reef in May.

The latest operation, close to Woody Island, which functions as China’s administrative centre in the South China Sea, is particularly provocative. As tensions with the US have mounted, China has placed anti-aircraft missile systems on the island and flown fighter jets there. Woody Island is to the south of major Chinese naval facilities on Hainan Island, adjacent to the Chinese mainland.

The Chinese Defence Ministry branded the US operation “illegal” and “intentionally provocative behavior,” and lodged a protest with the US. It accused Washington of being a “troublemaker when it comes to the stability of the South China Sea” and urged it to respect China’s “national sovereignty and security interests.”

The ministry warned that the Chinese military would increase its patrols, strengthen its defence capabilities and “resolutely defend national sovereignty and security.” Two Chinese warships, the Guangzhou and the Luoyang, shadowed the USS Decatur and demanded it leave the area.

The US naval operation took place just one day after Philippine President Duterte concluded his state visit to Beijing, during which he declared that he was “separating” the Philippines from the United States both diplomatically and militarily. In his bid for billions of dollars in Chinese aid and investment, Duterte played down his country’s territorial disputes with China in the South China Sea.

Duterte’s tilt towards China is in marked contrast to the foreign policy orientation of his immediate predecessor, President Benigno Aquino, who spearheaded the Obama administration’s efforts to confront Beijing in the South China Sea. Aquino had ramped up closer military ties with the US, including via a new basing agreement that has already opened up five Philippine bases to American forces, and adopted a confrontational approach in territorial disputes with China. At one point, he likened Beijing to the Nazi regime in Germany.

Duterte’s about-face is a significant blow to Washington’s “pivot to Asia”—an all-embracing diplomatic confrontation and military build-up against China over the past five years throughout the Asian region. Hillary Clinton, as Obama’s secretary of state, was central to the launching of the “pivot,” including by fueling tensions in the South China Sea through her declaration in 2010 that the US had “a national interest” in the disputed waters. The South China Sea is central to the Pentagon’s strategy for war against China, which includes a massive air and missile attack on the Chinese mainland, supplemented by a crippling naval blockade.