You might not notice Second Thomas Shoal from the air. The disputed atoll near the Philippines is a bit larger than Manhattan and sinks beneath the South China Sea’s surface at high tide.

But diplomats and military officials pay close attention to clashes near the shoal between Philippine and Chinese vessels. The fear is that an incident could turn deadly and prompt the Philippines to activate its mutual defense treaty with the United States.

That could lead to a nightmare: a war between the United States and China.

In one previous dramatic episode, Chinese Coast Guard vessels confronted Philippine Navy ones near the shoal in June, footage released by the Philippine military showed. Some Chinese sailors carried knives, and one Philippine sailor was injured.

Here’s what’s happening in the South China Sea, and why it matters:

The sea is valuable and contested.

The South China Sea has some of the world’s most productive fisheries, as well as shipping channels that carry about a third of global ocean trade. Surveys suggest that it may also contain large oil and natural gas deposits.

The sea’s features were mostly beyond “the administration and often the awareness” of governments until the late 19th century, the scholar Gregory B. Poling wrote in a recent book. But in the mid-20th century, Beijing laid claim to most of the sea with a sweeping, U-shaped boundary.