In recent years the People’s Republic of China has laid claim to ninety percent of the South China Sea, buttressing this claim by creating artificial islands with dredging equipment. These claims run roughshod over Beijing’s neighbors, which have competing claims. The discovery in 2016 that China had militarized these artificial islands was not exactly surprising, but just how useful are these islands in defense of China’s strategic goals?

China’s campaign to militarize the South China Sea began in 2009, when it submitted a new map to the United Nations showing the now-infamous “Nine-Dash Line”—a series of boundary dashes over the South China Sea that it claimed demarcated Chinese territory. Since then, China has expanded at least seven reefs and islets in the sea with sand dredged from the ocean floor, including Subi Reef, Mischief Reef, Johnson Reef, Hughes Reef, Gaven Reef, Fiery Cross Reef and Cuarteron Reef.

According to the Asia Maritime Transparency Initiative, Beijing has created more 3,200 acres of new land. China initially claimed its “territory” was being developed for peaceful purposes, from aid to mariners to scientific research, yet many of the islands now feature military-length airfields, antiaircraft and antimissile guns, and naval guns. Cuarteron Reef now has a new High Frequency early-warning radar facility for detecting incoming aircraft, a development difficult to square with a peaceful mission. Farther north, but still in disputed territory, China has installed HQ-9 long-range surface-to-air missiles on Woody Island.

On the face of it, China’s territorial grab and apparent turn away from former leader Hu Jintao’s concept of “peaceful rise” is hard to understand. It has alienated China’s neighbors and drawn in other powers, including the United States, India and Japan. One theory is that the country’s leadership may have calculated that securing a bastion for China’s sea-based nuclear deterrent may be worth the diplomatic fallout it created.

During the Cold War, the Soviet Union’s ballistic-missile submarines operated from two protective “bastions,” on the Atlantic side in the Barents Sea, and on the Pacific side in the Sea of Okhotsk. There, Soviet missile submarines could be covered by land-based air and naval forces to them from enemy aircraft, ships and attack submarines.

China’s nuclear “dyad” of land- and sea-based missiles relies in part on four Jin-class ballistic-missile submarines. China believes American ballistic-missile defenses threaten to undermine the credibility of its modest nuclear deterrent. In the Chinese view, this makes a protective bastion even more important.

The country’s geography leaves it with basically one ocean, the Pacific, for its own bastion. The Northern Pacific, with the U.S. Navy’s Seventh Fleet and the nearly fifty destroyers of the Japan Maritime Self Defense Force, is a no-go. The South China Sea, on the other hand, is bordered by a number of relatively weak states that could not pose a threat to China’s nuclear-missile submarines.

Sailing ships and flying aircraft through the South China Sea is one thing, but a permanent presence on the ground solidifies China’s hold on the region. It also allows, as the case of the HF radar on Cuarteron Reef demonstrates, the installation of a permanent sensor network.

The ports and airfields under construction will almost certainly grow to defend the region, with help from the mainland, from a complex antisubmarine warfare campaign designed to go after China’s seagoing nuclear weapons.

More surface-to-air missile batteries such as the HQ-9 and land-based antiship missiles are likely, if only to protect other military installations such as airfields and radar systems. Recent freedom-of-navigation operations by the United States and its allies will be used as a justification for heavier defenses. To paraphrase an old saying about bureaucracy, the military presence is growing to meet the needs of the growing military presence.

This points to the Achilles’ heel of China’s island garrisons: in the long run, they are impossible to defend. Unlike ships, the islands are fixed in place and will never move. Small islands cannot stockpile enough troops, surface-to-air missiles, food, water and electrical capacity to remain viable defensive outposts. As Iwo Jima and Okinawa demonstrated, there is no viable defense in depth for islands even miles across.

In any military confrontation with the United States, China’s at-sea outposts would almost certainly be quickly rolled back by waves of airstrikes and cruise missile attacks, devastating People’s Liberation Army facilities and stranding the personnel manning them. How China would respond to such an attack on its nuclear bastion is an open question that should be given serious consideration, as victory in the South China Sea may not herald the end of a campaign but a dangerous new turn in the war itself.

China’s military outposts in the South China Sea are a breach of Beijing’s agreement to not militarize the sea. Although the region itself has great strategic value, they are a poor defensive solution, prone to rapid destruction in wartime. China would be wise to consider the islands only as a temporary solution, until the People’s Liberation Army Navy has enough hulls to maintain a permanent presence in the region.

Kyle Mizokami is a defense and national-security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009, he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.

Image: An F/A-18E Super Hornet refuels. Flickr/U.S. Navy