How risky is the proposed summit between Donald Trump and Kim Jong Un? As America’s greatest modern presidents have found, summitry is an inherently high-stakes gamble. No matter how well scripted, events can take on a life of their own—or go dangerously awry. In negotiations, Mr. Trump is known for making snap decisions that amount to seismic policy shifts. It’s unclear whether this unpredictability would work in Washington’s favor or Pyongyang’s. Worse still, could a failed summit chill relations, or even propel the two sides to arms?
If...