Both critics and admirers have to admit that during his five years in office Japanese Prime Minister Shinzo Abe has earned a prominent place on the list of his country's post-World War II statesmen. Without doubt his most important legacy has been revising the 1946 Japanese constitution regarding self-defense, which finally won the support of the Diet in 2015. Abe has also had the political capital to get the Japanese defense budget raised to its highest level since World War II.

Abe's efforts at reviving the Japanese economy after two decades of stagnation, have been less successful. But certainly he has been forthright in confronting Japan's most severe geopolitical challenges in decades, not only with North Korea but with China's ongoing aggression in the East and South China Seas.

But his most important legacy may still lie ahead: namely, presiding over Japan's admission into the world's most exclusive club, the Five Eyes, as its Sixth Eye.

Who are the Five Eyes? They are the members of the oldest and most successful intelligence-gathering and intelligence-sharing alliance in the world. Formed after World War II, the alliance consists of the U.S., U.K., Australia, Canada and New Zealand (the term Five Eyes derives from the phrase "AUS/CAN/NZ/UK/US EYES ONLY," meaning information to be shared among the other members only). The alliance operated very successfully during the Cold War in gathering and sharing data regarding the Soviet Union, and it had a strong revival after 9/11 during the war on terror.

In effect, the treaty agreement allows each country to treat the others' intelligence agencies as if they were their own. Intelligence data, including satellite imagery, is shared without restriction except for specific exclusions at the request of either party. The alliance also defines what information or data is to be shared with third party countries, even (in the case of the U.S., U.K. and Canada) other members of NATO -- or with trusted allies such as Japan and South Korea.

Although the Five Eyes network was designed to share and monitor signals intelligence, i.e. intelligence drawn from electronic systems used by foreign targets, such as communications, radar and weapons systems -- it has come to mean much more. Being a member of the exclusive Five Eyes allows for mounting large-scale operations such as when the U.S., U.K., and Canada worked together to generate intelligence in support of their forces in Afghanistan under Operation Rampant Lion in 2008; the sharing of intelligence personnel and expertise; and joint high-level meetings of spy agency heads. Each agency remains accountable to its national government, and every Five Eye member has the right to opt out of any operation it feels is contrary to its national interest.

Even more vitally, being a member of Five Eyes is a major step toward building interoperability between the members' armed forces, especially the United States, which allows for seamless cooperation in joint military operations. It also facilitates joint securing of cyber networks. Three members of the club -- U.K., Australia, and Canada -- are also members of the Americans' National Technology and Industrial Base (NTIB), the Pentagon's global supply chain.

Japan already has a long history of cooperation with the Five Eyes in the intelligence field going back to the Cold War -- so much so that analysts already rank it informally as the Sixth Eye. But conversations with Five Eye officials, both past and present, make it clear formal Japanese membership would be very welcome. It would be hard to overestimate the value of Japan's systematic participation in planning and executing intelligence-gathering in places like the South China Sea as well as in mainland China and the Korean Peninsula. Japan could also share the Five Eyes burden of intelligence collection in Southeast Asia with Australia and New Zealand.

Japan itself would benefit tremendously by being able to contribute to setting the agenda for joint intelligence operations. It would expand opportunities for building interoperability of defense systems with the U.S. and other Five Eyes. At the same time, everyone would be happy to see Japan integrated into a defense industrial cooperation network such as NTIB.

This raises the question of why Japan hasn't been asked to join before. The Five Eyes don't consider themselves an exclusive Anglosphere club: France was asked to join in 2009, as has (according to reports) Germany. Both refused, largely out of fear that their own intelligence operations outside the Five Eyes networks might be compromised. It's unlikely Japan would have any such reservations. Indeed, Japan's new more proactive stance on the world stage makes it likely that any offer to join would be viewed more favorably now than ever before.

So what's necessary for Japan to join? A nomination by President Donald Trump would be a major step but not sufficient without the approval of other members. They, and the U.S., would need to be reassured about Japan's ability to handle the increasing flow of classified data outside its intelligence agencies, as part of its broader policies regarding defense industrial security.

It's an area where Japan has recently taken major steps, as a new Hudson Institute study has revealed; but more is still needed to achieve the "equivalence" standard that the other Five Eyes enjoy with the biggest Eye of them all, the United States.

Given the sensationalist revelations in WikiLeaks about Five Eyes covert operations like ECHELON, the global telecom eavesdropping operation disclosed by whistleblower Edward Snowden, the Japanese public would also need plenty of assurance. People would need to know that joining the Five Eyes club wouldn't pose a threat to privacy, but rather would be the keystone for future regional security, including for Japan.

In the final analysis, Japan deserves to be the Sixth Eye, and the Five Eyes deserve the benefits of Japan's membership. I myself have raised the issue with senior American and Japanese defense officials, and the response has been favorable. But the two people who could make the Sixth Eye reality are President Trump and Prime Minister Abe. For Trump, it would greatly increase his leverage in dealing with China. For Abe, formal membership would cap his efforts to build the "special relationship" with the U.S. similar to the one the oldest Five Eye member, the U.K., already enjoys. It would also earn him the laurels as Japan's most significant prime minister in the postwar era.

Arthur Herman is a senior fellow at the Hudson Institute.