AP Photo

A man in jeans and a jumper is standing in the road, waving his arms in brave defiance as bullets crackle around him. A few seconds later, he crumples and is loaded, bleeding, into a car to be taken to hospital. It's a few minutes of footage from the streets of Manama in Bahrain and the kind of incident that has become familiar in the last few months of Arab uprisings. But pause a moment, because this image of extraordinary, reckless bravery can become iconic in different ways to its many web audiences. Do we understand all of them?

Westerners see a political activist; some Sunni Muslims see a Shia troublemaker; and Shias across the Muslim world see a martyr. There is no more powerful a mobilising idea in Shia Islam than the martyr. For nearly one and a half thousand years,...