This morning, Japan time, President Donald J. Trump left Tokyo for Seoul, ending a two-day visit to Japan and a series of discussions with Prime Minister Shinzo Abe.

His agenda included a visit with Japan's emperor and empress, as well as a discussion with the families of Japanese citizens abducted by the North Koreans.

But the bulk of his time was spent with Abe, reflecting the personal relationship that has come to represent the alliance at a critical moment of transition.

This was the fifth meeting between president and prime minister. And it is only 10 months into the Trump administration. As Abe proudly noted, this is a historic level of policy coordination between the leaders of the two countries.

Japan's prime minister, and his team in MOFA, has worked hard to develop this relationship with the U.S. political transition.

It has not been easy, and while Abe's critics worry that he is overly solicitous of Trump, the escalating pressure on the alliance at the moment offers little alternative.

Indeed, much of the impetus for policy coordination has come from Pyongyang, and its accelerated program of missile and nuclear tests.

The opportunity for an early show of U.S. reassurance came during Abe's first visit to Washington in February this year.

Punctuating the many, many holes of golf Abe played with Trump at Mar-a-Lago was the North Korean missile salvo aimed at the Sea of Japan.

Abe's hastily organized press conference created the opportunity for Trump to assure the Japanese people that the U.S. was "behind Japan 100 percent."

Close consultations on North Korea have continued ever since, with 16 phone calls adding to the sense that Japan's prime minister was one of the U.S. president's closest foreign advisors on how to handle the growing crisis in Asia.

But Trump's visit also revealed the trouble spots for Abe, despite his personal ties.

Trump remains firmly convinced that the U.S. trade deficits with trading partners must go.

He has chastised other U.S. allied leaders publicly for their failure to offer the U.S. "fair and open" access to their markets.

German Chancellor Angela Merkel was Trump's target during the NATO summit, and again at the G-20, even though diplomacy might have dictated a more private approach to the host nation's leaders.

Similarly, Trump has pressed South Korean President Moon Jae-in to renegotiate the KORUS agreement, much to the displeasure of the U.S. Congress, even as tensions with the North intensified.

No U.S. president has used security protection for its allies so blatantly -- and publicly -- as this president.

Trump has not threatened Japan, however. Instead he has taken a different approach.

In his comments after the summit meeting with Abe, the president pointed out that Japan's trade deficit can be reduced by the significant purchase of U.S. weapons.

The trade-off was clear: "This will bring lots of jobs to America and lots of security for Japan."

Of course, Japanese security planners will welcome access to advanced missile defense systems and other weapons from the United States.

But a more troubling challenge for Japan -- and for Abe -- is in meeting the president's expectations that Japan will use these weapons to shoot down North Korean missiles. Perhaps the quiet discussions went beyond missile defense to include conventional strike capabilities, another possible addition to Japanese capabilities that many feel will make Pyongyang less likely to threaten Japan.

This first visit to Japan seemed to end well, and both governments worked hard to ensure that the prime minister's warm welcome and personal touch made this a comfortable stop for Trump as he looks ahead to a long and difficult itinerary in Asia.

Abe will have to put out a few fires at home in the wake of Trump's visit, however. Trade and investment will continue to loom over the horizon, as the president continues to be seen as unaware of the extent of Japanese business investment already in the United States.

While cooperation on North Korea seems assured, the expectation that Tokyo will shoot down missiles remains subject to the Abe Cabinet's continued stance that Japan will initiate the use of force only if its territory is threatened.

Once Pyongyang's missiles can reach the United States, however, this separation of Japan's security from that of the United States will become more of a problem for Tokyo.

And yet, it is not only the bilateral ties between Washington and Tokyo that matter. What comes next on the president's trip will certainly have repercussions for Japan.

Close coordination with South Korea is the backbone to defense and deterrence against North Korean belligerence, and the U.S. and South Korea presidents will need a good visit to settle the growing anxiety in Seoul about how to manage the North.

A strong but strategic management of the U.S.-China relationship is the foundation of peace in the region. Abe is hoping to jump-start high-level summitry with Beijing -- first with a Japan-South Korea-China trilateral in Japan in December attended by Premier Li Keqian and then hopefully with reciprocal bilateral visits by Abe and Xi Chinese President Xi Jinping next year.

A downturn in Washington's relations with Beijing could complicate Tokyo's diplomacy, and if a serious trade conflict between the two erupts, this could spill over to affect Japanese economic growth.

Shaping the Trump administration's Asia policy is Abe's goal, and he will see the president again at both upcoming multilateral meetings -- the APEC meeting in Vietnam and the East Asia Summit in the Philippines.

I expect Abe will continue to urge Trump to advocate for Asia's future, and some hope can be taken from the new language emanating from the Trump administration on a "free and open Indo-Pacific."

Working to ensure that the U.S. president is not isolated -- nor uninterested -- in an increasingly contested Asia is in Japan's best interests, and it seems like Japan's prime minister is quite willing to work at helping the U.S. president navigate the difficult currents ahead.

(Sheila A. Smith is Senior Fellow for Japan Studies at the Council on Foreign Relations. She is the author of Intimate Rivals: Japanese Domestic Politics and a Rising China (Columbia University Press, 2015) and Japan's New Politics and the U.S.-Japan Alliance (Council on Foreign Relations, June 2014).)