No matter what happens during the summit on June 12, it will be impossible to know immediately afterwards whether it's been a success.

The big picture: Trump himself has acknowledged as much, playing down expectations in his public statements and describing this as the first of a potential series of meetings designed to thaw relations and ultimately denuclearize the Korean Peninsula.

• Jeremy Bash, chief of staff to Leon Panetta when he was CIA Director and Secretary of Defense, told me “the only yardstick by which you can measure success is whether or not they get rid of their uranium enrichment, their plutonium program and their missile program."
• "Everything else is just pageantry," he added. "Of course that can’t happen on day one, but the only way to judge success is to see if that happens over time.”