For Kim, it may already have been achieved — so long as he can avoid an acrimonious collapse of the talks. A meeting with a sitting U.S. president is something his father and grandfather both sought in vain.

The calculations for Trump will be more complex. The U.S. has been trying to pressure North Korea into abandoning its nuclear ambitions for at least 25 years. In a series of hailed and failed agreements, Pyongyang committed itself first not to build nuclear weapons and then, once it did, to freeze, “abandon” or “disable” the program. By now it has a small arsenal of nuclear warheads and the missiles to carry them.

For those familiar with that history, expectations for success at the summit are set low: avoid a return to last year’s apocalyptic threats and start a longer negotiating process, without prematurely giving away U.S. troop deployments or sanctions leverage.