Reuters

TOKYO – When the U.S. Defense Secretary arrives in Asia this weekend, his biggest challenge may not be convincing China that America will give its full support to longtime ally Japan in the escalating dispute over islands in the East China Sea. His biggest challenge may be convincing Japan.

“There is a perception in Japan that the U.S. commitment is ambiguous,” says Yoichiro Sato, director of international strategic studies at Ritsumeikan Asia Pacific University in southern Japan. “If China thinks Japan will hesitate to respond or that America will hesitate, that will embolden the Chinese. It’s better that America sends a clear, explicit message now than have to respond to something worse later.”

On Friday, a group of six Chinese marine-surveillance ships entered territorial waters around the remote Senkaku Islands, which are claimed and administered by Japan as sovereign territory. It was the latest and most serious escalation in the dispute over the small but potentially valuable islands; they are also claimed by China, where they are known as Diaoyu.

Defense Secretary Leon Panetta is scheduled to meet separately with Japan’s Foreign and Defense ministers on Monday before continuing on to China and later New Zealand. It will be Panetta’s third trip to Asia in 11 months as the U.S. looks to rebalance its forces in the region in response to China’s growing military power and assertiveness.

The disagreement between Japan and China over ownership of Senkaku/Diaoyu, a group of tiny islets and rock outcroppings near Taiwan, has grown increasingly bitter. Though surrounding seabeds are believed to hold large deposits of oil and gas, the dispute hinges largely on issues related to Japan’s wartime and colonial period.

(PHOTOS: Anti-Japan Protests Hit China’s Capital)

The lightly armed Chinese marine-surveillance ships’ entry into Senkaku/Diaoyu territorial waters was an apparent display of Beijing’s displeasure at the purchase of the islands by Japan’s national government earlier this week. Though Tokyo insists that the purchase — from private owners in Japan — was necessary to preserve the islands in their current state, China views the transaction as illegal and an affront to its sovereign rights.

(Kyodo News reported Saturday that more than 60,000 Chinese citizens staged anti-Japan rallies in at least 24 cities to protest the purchase of the islands. These appeared to be the largest anti-Japan demonstrations in China in terms of participants and cities involved since the two countries normalized diplomatic relations in 1972. Some rallies turned violent, with protestors clashing with police and destroying Japanese-brand cars. The rallies appeared to have the tacit approval of Chinese authorities amid a flood of anti-Japan reports and broadcasts by official Chinese media.)

Officially, the U.S. takes no position on the Senkaku-Diaoyu dispute or the many other conflicting territorial claims that are upsetting the region. Under the U.S.-Japan Security Treaty, the U.S. is obligated to respond to any attack on Japan or its territory. Pressed to declare whether that security umbrella includes Senkaku/Diaoyu, U.S. officials stated publicly that the treaty applies to “all areas under Japanese administration” — a seemingly clear nod to Senkaku/Diaoyu.

But Sato says that’s not clear enough. The alliance also calls for Japan to take “primary responsibility” for territorial defense. That could give the U.S. a loophole to avoid confronting its most important trading partner and leave Japan on its own, he says.

“If Japan loses the islands and the U.S. doesn’t come to aid Japan, the credibility of not only the U.S. alliance with Japan but of all U.S. alliances globally would be severely harmed,” Sato says.

For an officially pacifist country, Japan has a deceptively large and powerful military. More than 250,000 of its men and women are in uniform, and its annual defense spending is the 6th highest in the world. Its maritime forces bristle with modern submarines and surface warships.

In 2010 Japan adopted a realignment plan to better protect its southwestern islands, which extend more then 750 miles (1,200 km) from Japan’s home islands. But it has a long way to go. While Japan’s naval forces are more than a match for China’s navy and armed patrol fleet, ground forces still have little capability to retake islands that China, if so inclined, might succeed in occupying.

About 40 Japan Ground Self-Defense Force troops are conducting their first-ever amphibious warfare training this month with U.S. Marines in Guam; the JGSDF plans to buy four amphibious assault vehicles — but not until next year. That’s too little, too late to help with Senkaku/Diaoyu.

The U.S., meanwhile, has more than 14,000 Marines stationed in Okinawa, many of them veterans of multiple tours of duty in Iraq and Afghanistan. The powerful U.S. 7th Fleet is homeported in Yokosuka, Japan, within easy range of the East China Sea.

But for all that, a direct confrontation at sea is unlikely, says Alessio Patalano, a Japan naval historian and East Asia security specialist at the Department of War Studies, King’s College London. He notes that the patrol vessels cruising Senkaku/Diaoyu belong to neither China’s navy nor its coast guard; instead, they are part of the Chinese marine-surveillance service, a largely civilian organization charged with environmental protection, scientific research, enforcement issues related to exclusive economic zones and similar duties.

“The PLA Navy is aware of its limitations, and they don’t want to get a beating from the Japanese,” says Patalano, who presented a series of lectures in Beijing and Tokyo this month. “The more likely scenario would be for China to insert special forces under cover of night, by parachute or other means. When the Japanese wake up in the morning and see Chinese soldiers on one of their islands, what do they do then?”

Brad Glosserman, executive director of the Pacific Forum CSIS in Honolulu, says there’s little doubt that the U.S. would respond if shooting were to break out between China and Japan. The key, Glosserman says, is to make sure the Japanese know exactly what they can count on from the U.S. — and what, if anything, they can’t.

“The U.S. will be there, because if we aren’t, our credibility is shot and the Japanese will never trust us again. That would transform the regional security environment, and the Chinese will think they have carte blanche,” says Glosserman. “But the problem is, do Americans and Japanese agree on what ‘being there’ means? Does that mean submarines? Surface warships? Helicopters with Marines rappelling to the ground? The Americans need to understand what the Japanese expect of them, because failure to do those things could cause big problems.”