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 revolutions 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 mobilization much easier.

If people are ready to come out on the street and protest, these media make it easier for them to organize and easier for the example of the protestors to spread. 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 U.S. 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 U.S. troops are leaving that country this year, and after that grim experience U.S. 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 U.S. 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 U.S. 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. The Arab oil exporters choose their customers on a purely commercial basis, and they have to sell their oil to support their growing populations. You don’t need to control them or threaten them to get oil from them; just send them a cheque. Besides, less than a fifth of America’s oil imports now come from the Arab world.

As for Israel, its military value to the United States has gone into a steep decline since the end of the Cold War. Nor does it need American protection: it is a dwarf superpower that towers over its Arab neighbours militarily. So remind me again: why, exactly, should the United States see “stability” in the Middle East as a vital national interest?

The revolutions of 1989 became possible when people in the Eastern European countries realized that the Soviet Union would no longer intervene militarily to preserve the Communist regimes that ruled them. Is another 1989 possible in the Arab world?

Well, the Arabs now know that the United States will not intervene militarily to protect the regimes that rule them.