NEWPORT, R.I. — In a speech in Tokyo Sunday, U.S. Defense Secretary Chuck Hagel made a not-so-subtle reference to China's aggressive behavior in the disputed Senkaku Islands, warning that countries cannot "redefine boundaries and violate territorial integrity and sovereignty of nations by force, coercion or intimidation," whether that be "small islands in the Pacific or large nations in Europe." Two days later, Hagel's Chinese counterpart, Defense Minister Chang Wanquan fired back: China, he said, has "indisputable sovereignty over the Diaoyu" -- as the Chinese call the islands -- while noting that the "Chinese military can assemble as soon as summoned, fight any battle and win."



Beijing's position on the islands is clear. But are the Senkakus dessert, or are they an appetizer?

If Chinese troops were to seize the Senkakus, might they also wrest the nearby Ryukyu Islands from Japan? It's not so far-fetched: Japanese strategists fret about how to forestall a doomsday scenario in the Ryukyus, the southwestern island chain that arcs from Japan's home islands southwest toward Taiwan.

Americans should worry as well. The southern tip of the Ryukyu Islands sits only about 80 miles east of the Senkakus. Unlike the uninhabited Senkakus, the Ryukyus host not only roughly 1.5 million Japanese residents, but also the U.S. Marine and Air Force bases that anchor the U.S. presence in the East China Sea. Occupying the Ryukyus would fracture the U.S. strategic position in East Asia -- separating U.S. forces based in Japan (to the north) from those at Bahrain, the other permanent U.S. hub in Asia, far to the west. At a bare minimum, U.S. ships and aircraft would have to detour around Chinese-held islands, waters, and skies -- incurring the additional time and costs longer voyages entail.

Island combat may seem anachronistic in this day and age, but in fact there are sound strategic reasons for China to take some or all of the Ryukyus. Currently, U.S.-Japanese forces are able to cordon off the East China Sea from the Western Pacific by fortifying the islands with weaponry able to strike at shipping and aircraft that venture within range, thus keeping Chinese ships from exiting or reentering. A near-seas People's Liberation Army (PLA) offensive, on the other hand, would guarantee access to the Western Pacific for Chinese warships and merchantmen -- balking allied efforts to wall off vital sea space.



Conquering the islands would pay offensive dividends as well. Emplacing PLA naval, air, and missile forces along the island chain would give the PLA a presence jutting out into the East China Sea. Forces based to Japan's south could threaten north-south movement along the Asian seaboard and well into the Western Pacific. Maritime control -- the ability to close shipping and air routes -- thus bestows political clout on Beijing.



PLA control of Okinawa, furthermore, would dislodge the U.S. Air Force and Marine Corps from bases there, loosening the United States' strategic position in Asia. Of course, this would also be a risky endeavor for China: 18,000 Air Force personnel call Okinawa home, and a Chinese attempt to take Okinawa -- which could cost many American lives -- would likely mean all-out war. Nevertheless, Beijing may calculate that it can strike hard and swiftly, handing Washington a fait accompli. Once Okinawa was in Chinese hands, the onus would fall on U.S. forces to retake it -- an unappealing prospect. China might gamble that U.S. leaders would shrink from paying the heavy price necessary to return.



And -- last but not least for patriotic Chinese -- a Ryukyus adventure would jab Tokyo in the eye. It would right what they consider a historic wrong: Imperial Japan's seizures of the islands from China in the 17th through the late 19th centuries. It would also announce China's return to the top of the Asian pecking order -- and do so in resounding style.



In short, grabbing the Ryukyus could look to Chinese eyes like a minor operation that would pay massively disproportionate dividends and hasten the end of U.S. dominance in the Western Pacific.



The logic, then, is compelling on many levels. Indeed, in May 2013, a group of scholars, analysts, and military officials converged on Beijing's prestigious Renmin University to debate the nation's claim to the Ryukyus. Most attendees apparently concurred that Beijing should make -- or at least threaten -- a claim on the islands.



While the Chinese Communist Party (CCP) distanced itself from the gathering, op-eds endorsing efforts to recover the islands have appeared in the People's Daily, a CCP mouthpiece, and the Global Times, a tabloid that likewise walks the party line. In short, Chinese scholars of militant leanings clamor for Beijing to undertake an East China Sea campaign. And the leadership is still permitting such views to be aired. Influential Chinese, then, are thinking about the unthinkable -- war with the U.S.-Japan alliance to overturn the U.S.-led system in Asia. Thus forewarned, the United States and Japan must think about it as well -- and arm themselves accordingly.



U.S. Pacific Command intelligence chief Capt. Jim Fanell confirmed in February that Beijing is girding for a "short, sharp war" with Japan in the East China Sea. PLA forces, prophesies Fanell, will make quick work of the Japan Maritime Self-Defense Force (JMSDF) before occupying "the Senkakus or even southern Ryukyu[s]." Since the United States is bound by treaty to defend Japan, U.S. forces would be drawn into the fight. Hence the PLA's emphasis on achieving a quick, decisive victory before U.S. forces could respond effectively.





Making war plans, of course, isn't executing them. But even as an exercise, it's important to think about tactics were China to proceed. How would an island-hopping campaign unfold?

Such a campaign will differ markedly from the last major effort of this type, the U.S. military's land/sea offensive through the South Pacific 70 years ago. First of all, the theater today is far more compact. The U.S. amphibious advance during World War II covered thousands of miles. About 3,000 miles separate the southwestern Pacific island of Guadalcanal, the campaign's starting point, from Leyte, where Gen. Douglas MacArthur's forces went ashore in 1944 to reclaim the Philippines. By contrast, the Ryukyu chain spans just over 600 miles, from Japan's southernmost home island of Kyushu through Yonaguni, the archipelago's southern terminus. The theater's layout will thus compress operations into confined spaces.



Second, a PLA offensive would likely proceed along multiple axes. MacArthur's forces fought their way basically along a single axis, lumbering along from the island of Guadalcanal westward toward the Philippine archipelago, with occasional help from Adm. Chester Nimitz's fleets steaming across the Central Pacific to the north.



By contrast, geographic proximity and the latest military technology open up new strategic vistas for PLA commanders. The southern Ryukyus -- the islands south of Okinawa, which perches at the island chain's midpoint -- fall under Japanese rule. But geography and military technology say they're contested terrain. Miyako-jima -- a Ryukyu island adjoining the Miyako Strait, a passage of choice into the Western Pacific for PLA seafarers -- sits about 330 miles from the mainland coast. But the island is also about 175 miles from Okinawa, over 500 miles from the southernmost tip of Kyushu, and farther than that from U.S. and allied naval and air stations. Miyako falls well within striking reach not just of PLA naval vessels but of missiles and tactical aircraft operating from mainland Chinese sites, which are hundreds of miles away. But protecting the island at such range from U.S. and Japanese bases will prove as burdensome for U.S.-Japanese forces as for the PLA.

So what does this all mean? Japanese who worry about a nightmare scenario appear to assume the PLA will grab the Senkakus, then rumble northward along the island chain, mounting amphibious assaults to wring every island from its inhabitants in turn. A linear campaign is certainly conceivable. But even if operations do unfold in sequence along the southwest-to-northeast axis, Chinese forces can mount flanking actions from the west employing land-based air, missile, and sea forces. They can concentrate power from the mainland to supplement expeditionary forces operating in the islands.

Advantage: Beijing.



As Fanell implies, however, Beijing might prove willing to settle for less than the entire Ryukyus. It yearns for easy access to the open sea. Rather than drive methodically along, taking island after island, the Chinese might simply grab one or two strategically located isles. Occupying positions on both sides of a navigable strait would let PLA forces guarantee passage between the East China Sea and the Pacific Ocean. A single safe entryway might satisfy Beijing. The PLA would have punctured the allies' offshore cordon -- and thereby fulfilled its paramount goal. It would also have ensconced itself in new positions just north of Taiwan, giving Beijing even more leverage vis-à-vis the island that it considers a rogue province, while securing access to natural resources found under the seafloor around the islands. The strategic gains from such a move would be substantial.



This is a dark picture to paint, but I do not counsel despair. Japan and the United States are not potted plants. As strategic sensei Carl von Clausewitz puts it, war is like a wrestling match on a grand scale. Imagine two sumo wrestlers grappling for advantage along the island chain, and you get the idea Clausewitz wants to convey. Both pugilists have options, and geography and technology cut both ways. If nautical geography bestows advantages on China to the south, it works for the United States and Japan in the northern reaches of the East China Sea. For one thing, advancing up the Ryukyus would bring PLA forces into close proximity with Okinawa and the Japanese home islands -- and with the robust military forces stationed there. The military balance will turn inexorably against PLA forces as they close on the northern isles.

Advantage: Tokyo and Washington.

Furthermore, PLA commanders cannot bypass Okinawa, the way MacArthur & Co. island-hopped over Imperial Japanese strongholds such as Truk Atoll and the Papua New Guinean town of Rabaul. Nor -- as American G.I.s of venerable years will attest -- is taking Okinawa from dug-in defenders easy or painless. Quite the opposite: landing into the teeth of enemy fire, fighting in the subtropical heat, and operating with short food, supplies, and ammunition make island warfare particularly forbidding. Just spool up the HBO miniseries The Pacific to glimpse its dangers and hardships.



Let me venture some guesswork about what the future holds: the southern Ryukyus, south of Okinawa, are indeed in greatest peril. They lie under the shadow of the Chinese coastline, fall roughly equidistant between the PLA and the islands' defenders, and are virtually unguarded. This is a mushy spot in the allies' defense perimeter. Chinese tacticians know this.



The northern Ryukyus are another matter entirely. U.S. and Japanese bases on Okinawa stand athwart any PLA advance into the northern Ryukyus. And any northerly offensive will carry Chinese forces into the teeth of Japanese and U.S. power in the home islands.



Given these assumptions, it makes sense to concentrate U.S-Japanese brainpower and resources on making Okinawa a bulwark against an island-hopping campaign, and on making the southern Ryukyus and adjoining seas no-go zones for PLA vessels and warplanes.



From a hardware standpoint, this may mean deemphasizing glamorous warships like guided-missile destroyers and helicopter carriers while investing in humdrum platforms like submarines and nimble, elusive, patrol craft packed with anti-ship missiles. The tactical setting, not ingrained preferences for big ships, should determine what the allies procure to fend off assault.



Admittedly, a Chinese lunge toward the Ryukyus remains unlikely. Whatever the strictly military rewards of such an offensive, Beijing has made next to no effort to prepare the ground politically. It has not declared "indisputable sovereignty" over the islands, as it has in the South China Sea, nor made them a "core interest" for which it's prepared to take up arms, like Taiwan. Nor has it dispatched Coast Guard ships to the Ryukyus' environs to stake its claim, as it has around the Senkakus. Such a campaign would come as a bolt from the blue.



Furthermore, the Ryukyus are inhabited -- unlike the Senkakus. Invading them would harm noncombatants, and would likely generate a massive international backlash against China. An island-hopping campaign, consequently, would entail serious diplomatic and economic costs -- over and above the losses PLA forces would incur in battle. Such factors may well give China's leadership pause.



But never underestimate the power of victory fever. Should China mount a successful operation against the Senkakus, the afterglow of military triumph might beguile the leadership into sending naval forces rolling up the island chain, in an attempt to solve China's maritime-access problem at one stroke. Beijing might believe it can wage a splendid little war, garnering vast dividends at minimal cost.



Tokyo and Washington must debunk such thinking. By hardening the Ryukyus against seaborne and airborne assault, they can drive up the costs of an island-hopping expedition to prohibitive levels. Advance precautions can make the islands look unappetizing indeed. And a cautious China is in everyone's interest.



James Holmes is a professor of strategy at the Naval War College. The views expressed here are his own.

(c) 2014, Foreign Policy