VLADIMIR PUTIN’S troop buildup along the border with Ukraine this spring garnered considerable international attention — which might have been his main objective. Less noticed has been a series of incremental escalations by Chinese forces in the Taiwan Strait and South China Sea. Both Mr. Putin and the regime of Xi Jinping are testing the Biden administration in its opening months. The difference is that while Mr. Putin is probably in it for the show, China is substantially advancing a strategy for establishing its dominance in East Asia and forcing Taiwan’s surrender.

In March, hundreds of “little blue men” — Chinese trawlers believed to be under the control of the People’s Liberation Army — suddenly appeared around Whitsun Reef, a boomerang-shaped scrap of the Spratly Islands that is inside the exclusive economic zone of the Philippines. Past incursions by what Beijing claims are fishing boats have led to the de facto seizure and fortification of other parts of the Spratlys — notwithstanding a 2016 ruling by an international tribunal rejecting China’s claim to the territories and most of the rest of the South China Sea. Philippine officials, with the public backing of the State Department, loudly objected to the new invasion. But at last report, half a dozen of the Chinese trawlers were still lashed together in the lagoon of Whitsun Reef, and others were posted nearby.

Chinese military forces, meanwhile, have been more overtly raising pressure on Taiwan. According to a count reported by the Wall Street Journal last month, warplanes have flown more than 260 sorties near the island so far this year, compared with a record 380 flybys in 2020. Last month a Chinese aircraft carrier cruised by the Taiwanese coast, and an official statement promised that “similar exercises will be conducted on a regular basis in the future.”

Few analysts expect offensive military action by China against Taiwan or in the South China Sea in the near future. But unlike Russia, which announced the withdrawal of its forces from the Ukraine border last month, the Xi regime is aiming for more than political or diplomatic points. As Michael Auslin of the Hoover Institution points out, it is steadily wearing down the Taiwanese air force, which must scramble every time warplanes from the mainland approach. If it keeps control of Whitsun Reef, it will have gained another pawn in its bid for control of the South China Sea and its vital sea lanes.

The Biden administration is not disregarding Beijing’s provocations. In addition to rhetorical backing for the Philippines, it has upgraded diplomatic contacts with Taiwan; though it has not yet gone along with suggestions that it end the long-standing policy of “strategic ambiguity” about whether the United States would defend Taiwan against a Chinese attack, it issued a statement saying its commitment to the island was “rock solid.” U.S. Navy ships have undertaken patrols in defiance of China’s South China Sea claims three times since Mr. Biden took office.

Unfortunately, such steps, and the nuturing of a “quad” partnership with India, Australia and Japan, are unlikely to reverse China’s slow escalation. A firm stand by the United States and its allies seems to have induced Mr. Putin to back off from Ukraine; deterring Mr. Xi is a much more complex challenge.

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