None of the many lists compiled two weeks ago, following the resignation of former Egyptian President Hosni Mubarak, put forward Libya as the next Arab dictatorship that was ripe for falling.

Yemen, Algeria and Jordan all looked pretty shaky, but Muammar Gaddafi's rule of iron was seen as too entrenched to be overthrown any time soon. Certainly he had the resources to smother any disquiet and the secret police to silence anyone who could not be bought off.

This week, with Tripoli a battlefield and at least a third of a country in the hands of the revolutionaries, he launched into an hour-long diatribe promising to fight until the "last drop of blood".

If Egypt and Tunisia were not enough, the Libyan uprising has served as the final proof that any attempt to predict which of the Middle East's tyrant class will be hit next is futile.

In retrospect, it all makes sense. Why should the heavily oppressed Libyans not have been quick to learn from their north African neighbours?

Each country has its own unique circumstances but some characteristics are shared by all: the freedom instinct, released like a flood breaking through a dam; the moment the fear of police repression is broken; and the speed with which the firestorm of revolt spread, fanned by the winds of Facebook, Twitter and camera-phones.

But what the uprisings have also exposed are the vulnerabilities at the heart of each of the regimes that ultimately brought about their downfall.

There was the corruption that personified the rule of President Ben Ali and his family and the deep hatred that festered between police and people in Egypt. Now we have discovered something we did not know about the Libyan power structure: it was built on an intricate balance of tribal loyalties bought off with Gaddafi's oil billions. The moment the tribes smelled weakness, some were quick to jump ship. Also Bahrain, another regime that surprisingly began wobbling this week, suffers from an endemic weakness. An ethnic minority has ruled over a majority that feels discriminated against.

It is too early to predict how Libya or Bahrain will play out, but despite Gaddafi's willingness to use degrees of force that Mubarak and Ben Ali were not prepared to consider, it is hard to see how his autocracy will survive for long. And it is almost inevitable that the new Arab revolution will spread. Similar vulnerabilities exist in each of the region's nations.

But the most intriguing question for the West, especially Israel and the US, is: which weakness will bring down Iran?