China looms as a significant factor in the deteriorating relationship. The U.S.-China trade war has strained ties between Washington and Seoul — South Korean mobile phone carriers are chafing at a U.S. demand that allies stop using Huawei equipment for their 5G networks. And even though China has punished South Korean businesses over Seoul’s 2017 acceptance of a U.S. anti-missile defense system, South Korea still wants to join China’s proposed multilateral trade arrangement (which does not include the United States) and will not support Washington’s Free and Open Indo-Pacific concept, which is designed to check China’s challenge to freedom of navigation in Asia.

This week, in another ominous sign of the U.S.-South Korea alliance weakening, the South Korean and Chinese defense ministers on the sidelines of a multilateral gathering in Southeast Asia inked an agreement to increase defense exchanges and establish military hotlines.

The collision of all these events could cause Trump to do the unthinkable by withdrawing U.S. troops from South Korea. Doing so would be in keeping with his long-held suspicions about the value of U.S. military commitments to allies, whom he thinks are free riders exploiting the U.S. security umbrella. Trump could use the failed burden-sharing negotiations with Seoul as an excuse to draw down or pull out troops. His list of indictments, beyond accusing South Korea of refusing to pay its fair share, would likely include criticism of South Korea’s delinking from the United States and Japan and its leaning toward China.