WHY now? Why revolutions in Tunisia and Egypt this year, rather than last year, or 10 years ago, or never? The protestors now taking to the street daily in Jordan, Yemen, Bahrain, Libya and Algeria are obviously inspired by the success of those revolutions, but what got the process started? What changed in the Middle East?

Yes, of course the Arab world is largely ruled by autocratic regimes that suppress all opposition and dissent, sometimes with great cruelty. Yes, of course many of those regimes are corrupt, and some of them are effectively in the service of foreigners. Of course most Arabs are poor and getting poorer. But that has all been true for decades. It never led to upheavals before.

Maybe the frustration and resentment that have been building up for so long just needed a spark. Maybe the self-immolation of a single young man set Tunisia alight, and from there the flames spread quickly to half a dozen other Arab countries. But you can't find anybody who really believes that this could just as easily have happened five years ago, or 10, or 20.

Yet there is no reason to suppose that the level of popular anger has gone up substantially in the past two or five or 10 years. It's high all the time, but in normal times most people are very cautious about expressing it openly. You can get hurt that way.

Now they are expressing their anger very loudly indeed, and long-established Arab regimes are starting to panic. The fall of Hosni Mubarak in Egypt, by far the largest Arab country, makes it possible that many other autocratic regimes in the Arab world could fall like dominoes. The rapid collapse of the communist regimes in Europe in 1989 is a frightening precedent for them. But, once again, why is this happening now?

' Social media' is one widely touted explanation, and the al-Jazeera network's wall-to-wall coverage of the events in Tunisia and Egypt is another. Both are plausible parts of the explanation, for the availability of means of communication that are beyond the reach of state censorship clearly makes mass mobilisation much easier. If people are ready to come out on the street and protest, the media makes it easier for them to organise. But this really does not explain why they are ready to come out at last.

The one thing that is really different in the Middle East, just in the last year or two, is the self-evident fact that the United States is starting to withdraw from the region. From Lebanon in 1958 to Iraq in 2003, the US was willing to intervene militarily to defend Arab regimes it liked and overthrow those that it did not like. That's over now.

This great change is partly driven by the thinly-disguised American defeat in Iraq. The last US troops are leaving that country this year, and after that grim experience US public opinion will not countenance another major American military intervention in the region. The safety net for Arab regimes allied to the United States is being removed, and their people know it.

There is also a major strategic reassessment going on in Washington, and it will almost certainly end by downgrading the importance of the Middle East in US policy. The Arab masses do not know that, but the regimes certainly do, and it undermines their confidence.

The traditional motives for American strategic involvement in the Middle East were oil and Israel. American oil supplies had to be protected, and the Cold War was a zero-sum game in which any regime that the US did not control was seen to be at risk of falling into the hands of the Soviet Union. And quite apart from sentimental considerations, Israel had to be protected because it was an important military asset.

But the Cold War is long over, and so is the zero-sum game in the Middle East.