Japan grows nervous about the US

The US and China have begun a protracted struggle that could shape global geopolitics for decades. That makes the US-Japan treaty alliance more important than ever for anchoring Asia's stability. President Trump should recognize that any development that undermines the American presence on the Korean Peninsula will also undermine Japan, and with it the whole region.

Japan towers above America's other Asian allies. South Korea, compromised by its fear of North Korea, is unduly susceptible to Chinese pressure. The Philippines is a badly governed, weakly institutionalized archipelago. Australia is in the Anglosphere but has a population of only 25 million and consequently lacks military and economic heft. Vietnam isn't democratic and lacks Japan's historical bond with the US.

With a population of 127 million, Japan is the world's third-largest economy and boasts one of the world's more high-tech and deployable militaries. Its navy, twice the size of Britain's, is critical to maintaining the balance of power in Asia. And don't forget that the US has 50,000 troops based in Japan as well as its only forward-deployed aircraft-carrier strike group.

Japan compares favorably with America's Western allies. Its internal politics are dependable in a way that Britain's and France's no longer are, while Germany is compromised by its energy dealings with Russia. Japan has rediscovered nationalism, but not the nasty, populist form familiar to some European countries.

Most important, Japan is a very lonely country. This works to America's advantage. Only Israel, another dependable US ally, is similarly isolated in its region. Japan has no real friends in East Asia. The wounds of Japanese aggression in World War II have not completely healed. Japan's crimes against humanity—mass cruelty and murder, but not genocide—weren't as bad as Nazi Germany's. This inhibited the country from fully reckoning with its war guilt. Until it does, Japan can never be fully forgiven by its Asian neighbors.

China increasingly looms as an existential danger to Japan, threatening to dominate the island nation's nearby seas and trade routes. The Korean Peninsula worries Japan, too. Japan would be dramatically weakened by the collapse of North Korea, Korean reunification, or both.

The 35-year Japanese rule over the peninsula (1910–45) is remembered mainly for its extreme harshness. Koreans, North and South, are united in their distrust of the Japanese; a reunified Korea would inevitably be anti-Japanese to a significant extent. Japan knows the 28,000 US troops stationed in South Korea cannot stay forever. Along with the threat from China, fear of major political change on the Korean Peninsula is the key factor fueling Japanese remilitarization.

In a contest between China and Japan for influence in any future Greater Korea, China would have the edge. It shares a contiguous land border with the North and is already the South's biggest trading partner. Japanese strategists have no choice but to consider a future in which China dominates the peninsula.

Nobody has made the Japanese more nervous than Mr. Trump. He has spoken cavalierly about Japan needing to defend itself, while setting in motion a sometimes chaotic process of negotiation with North Korea that has brought the two Koreas closer together. The US decision in October to cancel military exercises with South Korea has to make the Japanese doubly worried, even though Japan has its own disputes with South Korea over the sovereignty of the Liancourt Rocks islets and “comfort women” abused during World War II. If the US weakens its military ties with one ally, Japan recognizes, it may do so with another. And since Mr. Trump abruptly abandoned the Trans-Pacific Partnership, Japan has been within its rights to question the future of American leadership.

Then there's Taiwan. Because the nearby island was under Japanese occupation for 50 years (1895–1945), and was its first overseas colony, Tokyo has always taken a keen interest in the fate of Taipei. If the day comes—and Japan fears it's approaching—when the US can no longer credibly defend Taiwan against a Chinese attack, Japan will only feel more besieged and insecure.

Survey the region. Developments in China, the Korean Peninsula, and Taiwan mean that Japan has no one else to turn to but the US. The specter of a weaker or more unpredictable America could make Japan feel cornered—and become dangerous. There is little likelihood that Japan will develop nuclear bombs. But with its cutting-edge scientific base and a civilian nuclear-power program, the country could do so easily and quickly if it felt it had to. That experts are talking above a whisper about a nuclear Japan indicates that the situation in East Asia might be grave.

Neoisolationists believe Japan, like other US allies, should stand on its own two feet. But thanks to its deepening military insecurity, the Japanese are already toughening their armed forces. Unlike the Europeans, the Japanese don't need lectures. Japan's leadership wants to escape the shackles of its pacifist constitution, get its various armed services to work better together, and acquire amphibious assault vehicles, tanker aircraft, and much more.

That should be troubling. A Japan unbounded by a dependable US alliance system is a danger to itself and the region. Japan is the universal joint of American power in Asia; any weakening of the US-Japan alliance would signal the final eclipse of the American-led world. That's why any concessions to Kim Jong Un would come with a steep price—and the failure of the summit with North Korea may be a blessing in disguise.

This article originally appeared in The Wall Street Journal.