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.