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Once upon a time, Arab nationalism ruled the Middle East. Its doctrine saw Arab identity as the key to political success. Some regarded Islam as important; others were secular. Yet there was no doubt that national identity was in charge.

All Arabs should unite, said the radical nationalists who ruled in Egypt, Libya, Syria, Iraq and elsewhere, to destroy Israel, expel Western influence, and create a utopian single state in the region.

Instead, of course, the period was characterized by battles among the radical Arab states for leadership. The less extreme ones sought survival through a combination of giving lip service to radical slogans, paying off the stronger regimes, and getting Western help.

That era is over. We are now in the era of Sunni...