Three of seven artificial islands in the Spratlys are designed as military bases, the American military says. Among them, Subi Reef has a harbor bigger than Pearl Harbor, and another, Mischief Reef, has a land perimeter nearly the size of the District of Columbia’s, a submarine warfare officer in the United States Navy, Thomas Shugart, said in a paper issued this past week.

Together, the three islands could probably accommodate as many as 17,000 military personnel and support aircraft able to deter or counter an American military intervention, said Mr. Shugart, who is serving as a senior military fellow at the Center for a New American Security in Washington but writes as an independent analyst.

Scarborough Shoal, with a deep lagoon covering nearly 60 square miles, offers an even bigger prize as a potential Chinese military base. “The picture would become even worse were China to build and militarize a similar island base at Scarborough Shoal,” Mr. Shugart wrote.

Both China and the Philippines claim the shoal, which the United States used as a firing range during the Vietnam War. Until 2012, Chinese and Filipino fishermen operated there. Then China seized the shoal, and Chinese Coast Guard vessels have chased away Filipino fishermen ever since.

Just 150 miles from the Philippine coast and Subic Bay, where the United States stations fighter jets and naval vessels, the shoal is in a particularly strategic place.

Its conversion into a military base would enable China to project military power across the South China Sea from a triangle of bases formed by the shoal, the Spratly archipelago to its south and the Paracel Islands farther to the west and closer to the Chinese mainland, Mr. Shugart said.