AP Photo

Once again the world economy is on the brink. Only three years ago America was the epicenter of crisis. Today it is Europe. An enormous insecurity about the future has gripped ordinary citizens and investors around the world. Frustration and anger are spilling into the streets.

Once again the G20 must act to prevent a devastating slide into a deep contraction, if not depression, and avoid a damaging retreat into protectionism and competitive devaluation.

At the Cannes Summit, the G20 countries should recognize once and for all that, in today’s tightly linked global economy, no single country or bloc of countries is immune to...