If you are not making adequate progress in a negotiation, it may be rational to walk away from the bargaining table. But that is only the case if your adversary is more anxious to reach an agreement than you are.North Korea is by no means immune from external pressure; the claim that sanctions have no effect is contradicted by Kim Jong Un’s preoccupation with the economy in recent months.Nonetheless, Trump’s cancellation-by-tweet appears to reflect his erratic policymaking style, a misguided effort to link North Korea to economic negotiations with the Chinese trade talks and above all