When the rallying cry on the streets of Tehran turned from “Death to America!” to the stranger-sounding “Death to the Dictator!”, there was a great temptation to conclude that the days of the hardliners were numbered. Maybe they are and maybe not. But around the world, versions of the same question were being asked: Will the resort to raw repression work? Or will it eventually backfire, only widening the huge political breach that the election laid bare?

The history of repression to save regimes—or at least their leaders—is long. And every case is different: some regimes are brittle in the face of popular pressure while others are supple in adapting to it; some can use nationalism as their trump card, while for others, it is an Achilles’ heel. And if some regimes are simple tyrannies, the structure of Iran’s political system is especially complex and opaque.

Still, a common thread is clear: it is the security services on which the regime’s fate ultimately hinges. If they decide their best interests lie with the powers that they have protected, and that have protected them, they will stick it out. If they decide they are more likely to prosper under new leadership, power can collapse at the speed of a show trial.

There are a lot of gradations along that scale.

Twenty years ago this month, many inside and outside of China who witnessed Tiananmen Square confidently predicted the beginning of the end for the Communist Party. They were wrong. Two decades later, the party itself has changed radically enough—tossing aside its revolutionary ideology and replacing it with a social compact built on stupendous annual economic growth—that it remains secure, with its grip on power as solid as ever.

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