From a distance, the Chinese warship warned the American destroyer that it was on a “dangerous course” in the South China Sea. Then it raced up alongside, getting perilously close. For a few tense minutes, a collision seemed imminent.

The American vessel, the Decatur, blasted its whistle. The Chinese took no notice. Instead, the crew prepared to throw overboard large, shock-absorbing fenders to protect their ship. They were “trying to push us out of the way,” one of the American sailors said.

In 2014, the United States and China, along with other countries, signed the Code for Unplanned Encounters at Sea, which mimics aspects of the earlier pact with the Soviets and spells out protocols for confrontations.

But the code is voluntary, and it does not address the basic question of territorial waters and who can go where, said Collin Koh, a maritime specialist at the S. Rajaratnam School of International Studies in Singapore. “It’s more like a gentleman’s agreement,” he said.