Thursday marks a bitter anniversary in the Arab world. On Dec. 17, 2010, a Tunisian fruit vendor named Mohamed Bouazizi set himself on fire after the authorities confiscated his goods and beat him. The incident sparked an uprising that within weeks would topple Tunisia’s venal autocracy. Protests spread to Egypt, Libya, Yemen and Syria. Despots from Morocco to Mesopotamia felt the heat of popular anger. Many couldn’t withstand it.

Yet today the Middle East is less stable, and less hopeful, than it was before the Arab Spring....