America’s war in Afghanistan is winding down, but conflict elsewhere, including between China and Japan, now looks possible. Tokyo should get serious about its own defense.

After World War II the United States imposed a constitution on occupied Japan that formally forbade possession of a military. But the world is changing.

Imperial Japan is long gone. Most Japanese citizens seem prepared to defend themselves in a dangerous world.

Moreover, America no longer can afford to protect most of the known world from every threat. Despite the so-called “pivot” to Asia, U.S. forces will not remain forever.

Tokyo’s first duty is to protect Japan. Moreover, the Japanese government should promote regional security, cooperating closely with other countries in East Asia.

For years Tokyo’s defense spending only averaged 1 percent of GDP — and has not increased since 2002. Japan doesn’t need a large army, which would worry its neighbors. Most helpful would be missiles and missile defenses, as well as additional air and naval assets.

These issues have taken on new urgency in light of East Asia’s burgeoning territorial disputes. Japan is squabbling with South Korea over the Takeshima/​Dokdo Islands and with Russia over the Northern Territories/​Kuril Islands. In both cases Tokyo is contesting the status quo. The disputes are bitter, but unlikely to turn violent. More dangerous is Beijing’s challenge to Japanese control over the Senkaku Islands. These five islets have sparked naval clashes, aerial chases, activist flotillas, and domestic protests.

Prime Minister Shinzo Abe declared that the Senkakus are “Japan’s inherent territory” so “There is no room for diplomatic negotiations over this issue.” Indeed, he added, the solution necessitated, “if I may say at the risk of being misunderstood — physical force.”

Alas, Japan would not have an easy time if the two navies engaged. According to Michael Auslin of the American Enterprise Institute:”Tokyo would be forced to turn to the United States for support under the mutual security treaty.”

In 2010, then U.S. Secretary of State Hillary Clinton refused to take any position on territorial sovereignty, but explained: “we have made it very clear that the islands are part of our mutual treaty obligations, and the obligation to defend Japan.”

In November, then Defense Minister Satoshi Morimoto proposed updating the U.S.-Japan defense guidelines to include the Senkakus.

Unfortunately, issuing blank checks for the defense of weaker allies encourages them to behave irresponsibly — like Tokyo, which refuses to negotiate with China on the Senkaku Islands after politicizing the issue by purchasing three of the islands.

Washington should not put Americans at risk to guarantee other nations’ peripheral and contested territorial claims. More broadly, the U.S. should stop treating its allies as helpless dependents. Rather than augmenting American military forces in the Pacific, Washington should begin turning defense responsibilities over to others.

Tokyo is capable of doing more. Abe once criticized the Japanese Constitution for “failing to provide a necessary condition for an independent nation.” He indicated that his government will reconsider the informal 1 percent limit on military spending and may acquire amphibious units, ballistic missiles and strategic bombers.

Such increased military activity “may even cause Beijing to think twice about the cost of pushing its military and economic weight around East and Southeast Asia,” argues John Lee of Sydney University.

Tokyo also needs to forge better working relationships with its neighbors. Historical antagonisms loom large. Abe exacerbated these concerns by contradicting past apologies for Japan’s World War II impressment of “comfort women” to provide sexual services for its soldiers.

Abe, however, has begun to address these concerns. He appointed a special envoy to improve relations with Seoul and took his first overseas trip to Southeast Asia.

Japan’s ultimate objective should be to convince China that it has too many prosperous and nationalistic neighbors with expanding militaries to achieve primacy. The more Beijing asserts itself, the more surrounding states will respond.

Of course, the mere fact that peace is in every country’s interest does not guarantee peace. A nationalistic storm is building throughout the region. But the possibility of conflict is another good reason for the U.S. to stay out.

America has an interest in preventing any nation from dominating Asia, but no power, including China, will be able to do so in the foreseeable future. But the U.S. has no interest in acting as umpire for bitter territorial feuds throughout the region.

As Washington winds down more than a decade of fighting in Central Asia, some analysts would have the U.S. prepare for war in the Pacific. But Americans should reject this invitation for perpetual conflict. Japan and its neighbors should cooperate to counter Beijing’s geopolitical ambitions.