Washington and Seoul have reached tentative agreement on cost-sharing for the U.S. military presence in South Korea. The deal, known as the Special Measures Agreement (SMA), will increase South Korea’s contribution for the nearly 30,000 troops stationed there. The agreement is good news for regional security: There was concern that U.S. President Donald Trump might use failure to reach a deal as an excuse to withdraw those forces. The White House is clearly committed to increasing host-nation support. Japan must be ready for similar U.S. demands when Tokyo commences its own negotiations with Washington next year.

Seoul has paid about $800 million annually to support the 28,500 U.S. troops stationed on the peninsula to defend South Korea and deter North Korea. Trump has long charged U.S. allies with exploiting Washington — free- or cheap-riding on its alliances and forward-deployed forces. He has threatened to bring those forces home if those governments do not pay a larger share of the cost of their presence, and reportedly asked top advisers about the possibility of withdrawing U.S. forces from South Korea. Former Defense Secretary Jim Mattis pushed back vigorously against the idea; it is not clear what the current national security leadership thinks. The president recently said that he has “no plans” to withdraw U.S. troops from South Korea but added that “maybe someday” he would.

The previous SMA, which was in effect for five years (as most are), expired at the end of 2018. Trump wanted South Korea to double its contribution, to $1.6 billion. While South Koreans understand and appreciate the value of the U.S. presence, that support is not without limit. The government of President Moon Jae-in is left-leaning; some of his supporters would be happy to see the U.S. forces go home.

The recent agreement would increase the South Korean payment to just under $1 billion, the number that was reportedly a red line for Seoul. It is not certain if Trump will accept the deal. Rejection would be a public and humiliating repudiation of both his negotiators and a vital ally.

The new SMA is a one-year agreement. That means that the two governments will repeat the exercise soon; that is not a pleasant prospect given how bruising the last round of negotiations were. That short time frame also raises fears that Trump might use the U.S. troop presence as a bargaining chip in his meeting with North Korean leader Kim Jong Un later this month in Vietnam. At his last summit with Kim, Trump suspended U.S.-South Korea bilateral military exercises on an apparent whim. Steve Biegun, the U.S. special representative on North Korea, has said there have been no discussions with North Korea about any withdrawal of U.S. troops. At the same time, however, Biegun said the United States is “prepared to discuss many actions that could help build trust” between Washington and Pyongyang.

The prospect of another rash move by Trump moved U.S. lawmakers last year to insert a provision into the National Defense Authorization Act that would require the president to get certification from the secretary of defense before he could reduce the number of U.S. troops in South Korea to below 22,000.

Japanese security planners are pleased to see a new agreement. The stunning developments last year — the Singapore summit between Trump and Kim and the decision to suspend exercises — hammered home the importance of the Korean Peninsula to Japan’s own security. Strategists and officials repeat that Japan and the peninsula are a single operational theater; the two countries’ security is deeply integrated. The flow of personnel and materiel through Japan in the event of a conflict makes this country essential to the defense of South Korea. A strong, stable and credible U.S.-South Korea alliance is critical to Japan’s peace and stability.

The Japanese government must be concerned by the precedent set in the U.S.-South Korea negotiations. Japan’s host-nation support agreement expires in 2021. Negotiations on the next package will begin late this year and while Tokyo spends considerably more than Seoul on U.S. forces — about $1.6 billion annually — Trump has made clear that he wants more from all allies. Japan is also shouldering the cost of building a new base for U.S. Marines in Okinawa, which makes a substantial increase in its payments a nonstarter.

Tokyo should not hesitate to remind Washington that the U.S. military presence here is not just for Japan’s benefit, and matters greatly to U.S. power and influence in the region. This is, after all, an alliance, and like the one with South Korea, both partners gain significantly from their partnership.

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