Last month, the Philippines—an ally with whom America has a defense treaty and security commitment—was forced to withdraw a coast guard vessel it had anchored at Sabina Shoal, an atoll well within its exclusive economic zone that China claims as its own. Beijing besieged the shoal to block Manila from resupplying the ship it had placed there to prevent a Chinese takeover. With the ship badly damaged by the ramming it endured, and some of its crew members in need of medical attention, Manila had little choice but to abandon the shoal, which is now vulnerable to Chinese occupation.

The saga over Sabina Shoal came on the heels of a nearly yearlong standoff over the Philippines’ efforts to resupply personnel stationed on a crumbling World War II-era vessel beached on Second Thomas Shoal. That standoff ended after the two sides entered a “provisional agreement” that enabled the Philippines to resupply the ship.

That Beijing should dictate how Manila can operate within its own exclusive economic zone is outrageous. That the U.S. didn’t come to its ally’s defense as Chinese ships rammed holes into Philippine vessels is unconscionable.

During the height of the drama over Second Thomas Shoal, Biden reiterated that if Beijing crossed the threshold into armed conflict, America would come to Manila’s defense. It was an important reminder of Washington’s commitment, but such statements are the equivalent of giving China a green light to continue engaging in activities short of open warfare.

Biden should have known better after what he experienced as President Barack Obama’s vice president. In 2012, the Obama administration watched China illegally seize Scarborough Shoal from the Philippines. The next year, Beijing started building artificial islands to fortify its claims over several other rocks and underwater shoals in the contested Spratly Islands, sending the entire region in an uproar.

Obama protested to no avail, then warned Beijing not to militarize the islands, which it proceeded to do with no consequences.

The fact that China and the Philippines aren’t shooting at each other might partially explain the administration’s false sense of security. But Beijing’s incursions are part of a sophisticated strategy of gray zone warfare designed to extract concessions without the costs and risks associated with war. If they are allowed to continue, the best-case scenario will be that Beijing gradually occupies much of the territory it claims in the South China Sea without firing a shot, strengthening its position in one of the world’s most strategic waterways and making the region far more dangerous for the U.S. and its allies.

Even more worrying is the ever-present risk of war resulting from these incursions. Philippine President Ferdinand Marcos Jr. has publicly warned that any Filipino death resulting from China’s actions would be “very close to what we define as an act of war.” There have been a frightening number of encounters over the past year that could have easily led to such an outcome. Indeed, the transition from gray zone incursions to all-out war appears to be just one miscalculation away, and if that happens, the U.S. will find itself in a hot war with its most capable adversary. More must be done to ensure this outcome never occurs.

To preserve peace, the next U.S. president will need to abandon Biden’s timidity and actively push China back from the precipice of war. This will require intense diplomacy of the sort rarely seen since the end of the Cold War. It will be unpleasant, but that’s the nature of the brinkmanship Beijing is engaged in.

It may also require physically intervening to call China’s bluff and force it to stand down. For example, the U.S. should strongly consider sending Coast Guard or Navy vessels to accompany Philippine resupply missions or to escort a Philippine ship to replace the vessel that withdrew from Sabina Shoal last month.

Such actions would not be overly provocative, provided they are carried out in a professional manner that is designed not to needlessly humiliate Beijing, but they would give China a powerful ultimatum—pick a fight with the U.S. or back down. Beijing would choose the latter; the last thing it wants is to overplay its hand and end up in a war with the U.S.

China is engaged in a high-stakes game of chicken in which the side that flinches first loses. This works to its advantage when it goes head-to-head against the Philippines, but with the U.S. firmly on its ally’s side and willing to mobilize to show its support, the tables will turn and China will be forced to back down. But it won’t do so until it’s sure the United States is serious.