The strongest military alliances tend to be tempered on the battlefield, strengthened in combat against a mutual adversary. There, political bonds and effectiveness of military interoperability are put to the test.

For decades following the end of the postwar Occupation, the Japan-U.S. alliance had no such test — certainly nothing that required mass mobilization and coordinated response with such high stakes involved. That is, until Japan experienced a devastating earthquake in 2011 off the coast of Tohoku, triggering a tsunami that wiped out the northeastern coastline and sparking the nuclear disaster in Fukushima.

The allied response to the triple disaster on 3/11 through Joint Task Force Tohoku on the Japan Self-Defense Forces (SDF) side and Operation Tomodachi on the U.S. military side was unprecedented. Twenty-four thousand U.S. service members, 189 aircraft and 24 naval vessels joined the 106,000 SDF personnel in a weeks-long operation to stanch the damage and help put the nation back on its feet. While the two countries’ forces had exercised together before, there was nothing they had done that could compare to this.

I was not posted in Japan when the triple disaster occurred, but I have personal context for what it meant for the alliance. I spent the better part of my life living on U.S. military bases in the country before the disaster, and I worked in the Japanese government and for U.S. Forces, Japan as an alliance manager for most of the decade following it.

It is difficult to describe just how important 3/11 was in evolving the alliance, and its legacy lives on in ways that have political and practical impacts on the Japan-U.S. security relationship to this day.

One must remember that the fear of abandonment is an ever-present worry in alliance relationships. In the case of the triple disaster, the nuclear element alone gave many would-be contributors pause. Further, the United States had no legal obligation to respond; after all, there is no article in the 1960 Mutual Security Treaty that obligates the United States to defend Japan against natural disasters.

Yet, when disaster struck, U.S. forces were there en masse to support their host nation and ally. Obviously, a natural disaster is not the same as an armed attack situation, but it nevertheless sent an important signal to the Japanese government, the SDF, and the public about U.S. commitment to Japan.

The triple disaster and allied response also produced many lessons for the security partners. Operation Tomodachi required an unprecedented level of coordination between the U.S. military, U.S. government, SDF and Japanese ministries and agencies. It even included U.S. military personnel participating in daily meetings chaired by then-Minister of Environment Goshi Hosono and working directly with members of the Fire and Disaster Management Agency, Ministry of Land, Infrastructure, Transport and Tourism and others beside the traditional counterparts in the Ministry of Defense and Ministry of Foreign Affairs.

This coordination, while robust, was also imperfect, and there were inefficiencies and process obstacles along the way. While there is now much literature discussing all those problems, the important point here is that the lessons learned from that experience now influence the Japan-U.S. alliance through the security partners’ planning, training and exercising.

This is not purely academic either, as the allies now engage in new types of exercises, such Japan’s annual “Joint Exercise Rescue” and a biennial interagency coordination exercise that involves U.S. forces, the SDF and Japanese ministries and agencies. That level of cooperation would not have been achievable prior to 3/11.

Something else happened in the bilateral response to 3/11: it widened the aperture for what the allies can and should do in non-traditional security cooperation. This is critical because there are strict limitations on the missions the SDF can legally execute abroad, and those limitations prohibit the SDF from engaging in the traditional combat roles that America’s other allies fulfill.

However, in an era marked by “gray zone conflict” and great power competition, cooperation in non-traditional security missions like humanitarian aid/disaster relief is critical in advancing the interests of the two allies. The 3/11 disaster demonstrated that they could do it in meaningful ways, and the allies have communicated and cooperated in multiple disaster relief operations since, including Operation Damayan in the Philippines (2014), the international disaster relief operation in Kathmandu (2015) and JTF Chinzei in Kumamoto (2016).

All this eventually became institutionalized in alliance designs. Just two years after the triple disaster, the allies decided to renegotiate the Guidelines for U.S.-Japan Defense Cooperation, a framework for roles, missions and capabilities that has been in place since the last revision back in 1997. While the decision to revise the Guidelines was predicated on broader strategic objectives, the document concluded in April 2015 echoes much from the response to the triple disaster.

For example, the 2015 Guidelines call for the establishment of a “new, standing Alliance Coordination Mechanism.” That was in part because the previous “Bilateral Coordination Mechanism” hit bureaucratic obstacles when the allies tried to employ it in response to 3/11.

The “BCM” was tied to defense-only scenarios, and thus the U.S. military and SDF had to employ an ad hoc variant of it in Operation Tomodachi. Such is no longer the case, as was demonstrated in response to the April 2016 Kumamoto earthquakes when the Alliance Coordination Mechanism was employed for a smaller but significant bilateral disaster relief operation.

The new Guidelines also specifically called for the allies to include disaster relief operations as part of the mandate for the alliance. Importantly, there is not just a section on domestic disaster relief, but a provision for cooperation in international disaster relief missions as well.

It may seem like a lot of stock to put in a single document, but the 2015 Defense Guidelines is the new charter for the allies in terms of expected roles and functions. If precedent holds true, we can expect these Guidelines to be in place for about 20 years without amendment. This means that those lessons from 3/11 that were woven into the most recent Guidelines will continue informing alliance operations and designs into the 2030s.

None of this has yet to cover the human element of 3/11’s legacy on the alliance. I recall so many of my contemporaries in the SDF who joined the service because of what they saw growing up in response to the Hanshin earthquake in 1995. That event left an indelible mark on the public, and now with 3/11, the image in collective memory is that of U.S. and Japanese allies working together to recover Japan from trauma in an operation literally called, “Friendship.” It is impossible to define what impact that will have, but it would be foolish to underestimate it.

The legacy of 3/11 will continue to live on in the Japan-U.S. alliance. While the allies have many topics to discuss when the newest U.S. Secretaries of State and Defense visit their counterparts in Tokyo next week, inevitably, the 10th anniversary of the triple disaster will make its way into the discourse. Rightfully so, the “2+2” participants will remind us of the legacy of 3/11, the impacts of which have been ever-present in the alliance since that fateful day ten years ago.

Michael MacArthur Bosack is the special adviser for government relations at the Yokosuka Council on Asia-Pacific Studies.

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