One of the duties of a superpower is playing the role of therapist for your friends—which is precisely the task the U.S. performed this week to lower the temperature in a crisis between two crucial allies, Israel and Turkey.

The counseling began before dawn on Monday, shortly after Israeli commandos boarded a flotilla of six ships trying to run a blockade to deliver supplies to Palestinians in the Gaza Strip, and killed eight Turks in the ensuing fracas.

The...