It’s harder than it looks to get out of Korea. U.S. President Donald Trump already found that out in February, when, according to an NBC News report, the heated intervention of chief of staff John Kelly deterred him from attempting to order the withdrawal of all U.S. forces from South Korea. Trump may be taking comfort in the new developments on the peninsula, where South Korean President Moon Jae-in has praised him as a worthy recipient of the Nobel Peace Prize. But an end to the U.S. presence is still a long way off.

The smiles and jokes between Moon and North Korean leader Kim Jong Un at the inter-Korean summit on Friday were followed up by the joint Panmunjom Declaration, promising a nuclear-free peninsula and a formal end to the Korean War. Yet we’ve heard such peace pledges before, and it’s worth treating this latest example with skepticism. Finding a route toward a Korean peace treaty would be difficult under the best of circumstances — and, even then, the presence in South Korea of U.S. military forces, including nuclear-capable aircraft and ships deployed during regular military exercises, poses a problem with no clear solution.

Removing those forces from the peninsula has traditionally been treated as a condition for peace. True, North Koreans have shown more flexibility on the issue in recent years; there was no direct mention of U.S. forces in the Panmunjom Declaration, and Moon previously stated that Kim would not insist on the withdrawal of U.S. troops. But, eventually, the Trump administration will need to face a disruptive challenge it has yet to even acknowledge: developing a plan either for troop withdrawals or a significant change in the status of any troops that remain.

If a formal peace treaty were signed, as Time reporter Charlie Campbell correctly observes, it would undermine Washington’s argument for its continued military presence. As Christopher Green, a senior researcher at the International Crisis Group, tells Campbell: “There would be voices raised with the question: why are the U.S. troops still here if we have a peace regime in North Korea?” Trump is already on record as saying South Korea should no longer get a “free ride” from the United States and should defend itself.

While Trump may be inclined to make a sudden, grand gesture to remove U.S. troops, that has proved a hard task in the past. The most notable example occurred under President Jimmy Carter, who upon entering office in January 1977 quickly moved to implement his campaign promise to remove all U.S. ground combat forces of the 2nd Infantry Division from South Korea. However, by July 1979 his troop withdrawal policy was dead, killed by resistance within the U.S. foreign-policy establishment and Congress, as well as among regional allies. Carter’s experience carries powerful lessons for any plan to demilitarize the peninsula today. ...