Over the past 15 years, China has expanded its once-minimal military presence in the South China Sea into a significant one.

Beijing has laid claim to nearly all of the strategic waterway, a vital shipping lifeline for the global economy that is rich in energy and fishery resources. China has used nonmilitary assets such as its Coast Guard, fishing vessels and maritime militia to bully its neighbors, blockade their ships and build Chinese military bases on disputed islands.

America is partly to blame. It has condemned China’s behavior, but, eager to avoid escalation, has consistently refrained from standing up militarily, which has only further emboldened Beijing. A new approach is needed. The United States must take real action to strengthen alliances and confront China before it eventually takes control of this hugely important body of water without firing a shot.

Like any unchallenged bully, China has become increasingly aggressive. Last month, Chinese Coast Guard personnel attacked a Philippine supply vessel with axes and other crude weapons — Manila says a Filipino sailor and several others were injured — in one of the worst acts of violence between China and its rivals in the South China Sea in years. The incident took place near the Sierra Madre, a rusting World War II-era ship that the Philippines had beached 25 years ago at Second Thomas Shoal to assert its territorial claim. The shoal lies about 120 miles off the Philippine island of Palawan and is well within the nation’s exclusive economic zone.

China also had past territorial confrontations in the South China Sea or other waters on its periphery with Vietnam, the United States, Australia, Japan and Taiwan. In 2012, China took control of the disputed Scarborough Shoal from the Philippines, and run-ins between China and the Philippines have grown in number and intensity in recent years. In late May, President Ferdinand Marcos Jr. of the Philippines warned that any Filipino deaths caused by a “willful act” by a foreign force in the South China Sea would be “very close to what we define as an act of war.”

Concern has grown in Manila, Beijing and Washington that tensions in the South China Sea — perhaps even more than Taiwan — could trigger a conflict with China.

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