Defying precedent and conventional wisdom, President Trump says he’ll meet in May with North Korean dictator Kim Jong Un. Mr. Trump wants a sustainable deal that leads to North Korean denuclearization. The president’s critics scoff, and even his supporters are rightly skeptical. But Mr. Trump has conditions: His policy of maximum sanctions pressure will remain in place, Pyongyang must commit to the goal of denuclearization upfront, and it must refrain from missile or nuclear tests during talks. That may give him some leverage.

But...