Not Obama, not Bush, and not Twitter, in other words, but years of work and effort lie behind the public display of defiance—and in particular the numbers of women on the streets. And their presence matters. For at the heart of the ideology of the Islamic republic is its claim to divine inspiration: The leadership is legitimate, and in particular its harsh repression of women is legitimate, because God has decreed that it is so. The outright rejection of this creed by tens of thousands of women, not just over the last weekend but over the last decade, has to weaken the Islamic republic’s claim to invincibility in Iran and across the Middle East. The regime’s political elite knows this well. It is no accident that the two main challengers to President Mahmoud Ahmadinejad in the Iranian election campaign promised to repeal some of the laws that discriminate against women—and no accident that the leading challenger, Mir Hossein Mousavi, used his wife, political scientist and former university chancellor Zahra Rahnavard, in his campaign appearances and posters.

The Iranian clerics know that women pose a profound threat to their authority: As activist Ladan Boroumand has written, the regime would not bother to use brutal forms of repression against dissidents unless it feared them deeply. Nobody would have murdered a young woman in blue jeans—a peaceful, unarmed demonstrator—unless her mere presence on the street presented a dire threat.