The Middle East's descent into extreme violence — with mass killings of Muslim Brotherhood demonstrators in Cairo followed closely by Bashar Assad's use of chemical weapons in Syria's civil war — has dashed the hopes raised by the Arab Spring in 2011. The question now — and in terms of the future — is how to account for what is shaping up to be a profound historical failure.

In the 1990s, when communist regimes collapsed in Central and Eastern Europe, and dictators fell in Latin America, Sub-Saharan Africa and Southeast Asia, the Arab world stood out for its lack of popular, anti-authoritarian movements and developments. And, while the "Arab Spring" demonstrations in 2011 brought down or seriously challenged dictators in Tunisia, Egypt, Libya, Yemen and Syria, the result has been instability, violence, and civil war, not democratization. Why?

The Arab Spring did not affect all 22 Arab countries equally. The regimes that were brought down, or challenged, were military dictatorships cloaked in republican garb. None of the dynastic monarchies, some of them far more repressive (like Saudi Arabia) were confronted by serious popular challenges, with the exception of small Bahrain, owing to a sectarian divide between its Shiite majority and Sunni rulers.