Vance Serchuk, a former foreign policy adviser to Sen. Joseph Lieberman, is a Council on Foreign Relations-Hitachi international affairs fellow, based at the Canon Institute for Global Studies, in Tokyo. He writes a monthly column for The Post.

As the Obama administration grapples with North Korea, U.S. officials are investing considerable hope in securing Chinese cooperation. If that support fails to materialize — while Beijing continues to display unhelpful assertiveness on issues such as cyberspace and the South China Sea — concerns are likely to deepen in Washington about the kind of great power China is proving to be.

Here in Tokyo, such worries about China are already well-entrenched — and provoking as much anxiety as are developments on the Korean Peninsula. But they are also the source of something paradoxically hopeful: an impetus, albeit still tentative, for reform and renewal.

In the past two decades, Japan has become all too synonymous with decline, hobbled by anemic growth, dysfunctional politics and death-spiral demographics.

But, to paraphrase Samuel Johnson, nothing concentrates the mind like a noose, and for the Japanese, China’s recent assertiveness — in particular, its continuing dispatch of ships and aircraft to a group of East China Sea islands (called the Senkakus by the Japanese and the Diaoyu by China) — has previewed the future they face if Beijing keeps rising and Tokyo keeps falling. It is an East Asia, many here fear, that will be governed not by the rules-based international order the United States has nurtured but rather by Thucydides’s dictum in the Melian Dialogue: “The strong do as they can, and the weak suffer what they must.”

Behind closed doors, as a result, Japanese leaders and officials speak with new urgency about the necessity of painful and long-deferred reforms — including structural, economic and defense-related measures — that, until recently, seemed politically infeasible.

Prime Minister Shinzo Abe’s decision to join negotiations for the Trans-Pacific Partnership, which required going against deeply entrenched interests in the governing Liberal Democratic Party, is one example. There are encouraging signs that more will follow. While no one is eager to say so publicly, fear of being eclipsed by China is a major reason this decisiveness is now possible.

Of course, no one in Tokyo welcomes deteriorating relations with Beijing. Contrary to cliches about Japan’s resurgent nationalism, leaders have sought to contain and deescalate the confrontation over the Senkakus, exercising caution.

But Tokyo is right to worry about the implications of the growing gap between Chinese and Japanese power, and the United States has an overwhelming interest in a Japan that gets off the sidelines and back on its feet — economically, diplomatically and militarily.

In fact, nothing would bolster the Obama administration’s rebalance of East Asia more than a reinvigorated Japan. While most of the Asian countries that China has tried to intimidate in recent years have been developing powers — considerably weaker and poorer than Beijing — Japan is still the world’s third-largest economy and a high-tech powerhouse that, despite spending less than 1 percent of its gross domestic product on defense, possesses one of the most advanced militaries in Asia.

This means Japan is among the very few Asian powers with considerable untapped capacity not only to shoulder a greater share of its own defense but also to help the United States reinforce broader regional security — provided it can summon the political will to do so.

Of particular importance will be the raft of defense reforms the Abe government is likely to take up later this year. These include steps that many U.S. strategists have long sought, such as a reinterpretation of the Japanese constitution that would allow Tokyo to engage in “collective self-defense” with Washington; the creation of a national security council to better manage decision-making; and, possibly, a boost in spending on the Japanese Self-Defense Forces beyond the modest increase adopted this year.

The Obama administration has responded to Chinese-Japanese tensions over the Senkakus with appropriate caution, expressing support for its allies in Tokyo while urging “cooler heads to prevail.”

At the same time, the next 12 to 24 months could prove a rare window of opportunity to take the U.S.-Japan alliance to the next level. Washington should strongly encourage the tough reforms that will allow Tokyo to play a larger role on the world stage.

To be sure, Japan’s structural problems, particularly in its economy and demographics, are daunting and deeply rooted. Even with China providing an inadvertent push, it is far too early to judge whether the Abe government will prove capable of overcoming these — the real key to long-term renewal.

But even a partial reversal, or deceleration, of Japan’s decline could make a big difference to the Asian balance of power and to the U.S. position in the region.

Legend has it that the commander of Japan’s fleet during the strike on Pearl Harbor, Adm. Isoroku Yamamoto, warned afterward , “I fear all we have done today is awaken a great, sleeping giant.” The quote is likely apocryphal, but its strategic insight is not — as China’s leaders may discover, should they continue antagonizing their neighbor to the east.