Why and when do People's Revolutions succeed. I would argue, that -- for all very real differences between countries, cultures and history -- it really comes down to two factors:
1. Will the military, or other well armed internal security forces, kill large numbers of their own people?
2. Will the working class, actual laboring forces, go on general strike in support of the revolution?
3. A third predictive factor is whether food prices are rising.
In Tunisia and Egypt (so far) the answer is no, just as it was (ultimately) in eastern Europe in 1989 period, the Phillipines and perhaps the best analogy to Egypt which is Indonesia.
So far Iran is more like China of 1989. The military and other security forces are backing the oppressive regime against what starts as a predominently urban and relatively elite people's revolt. Hopefully the world media, Al Jazeera included, will provide honest complete ongoing reporting of the hypocritical nasty regime in Iran too.
Successful people's revolt does seem to be more likely to succeed when there has been built up over time a strong alternate civic society, that can then help promote the mass movement. But that existed in eastern Europe for decades prior to 1989, but failed in 1956 and 1967.
At the end of the day, push comes to literal shove, the difference is the willingness of the military to mow down large numbers of their own people.
And the other criticial factor, is whether the workers identify with the revolutionaries or the regime. Many working class people, especially if they are from areas and population separate from the those formenting revolution, may patriotically identify with the regime as representing their country and stability. We saw this in the use of miners and other workers to initially put down the revolution in Romania. Closer to home we saw a parallel this situation with many working class people in the U.S. backing the governement position in the U.S. during the Vietnam era.
Revolutions of just young people and students have little staying power or generalizable support, are easy to put down. Even if it is supplemented by the unemployed, this is less dangerous then if working people shut down the economy, which is a real threat to both the regime and the elite.
In Egypt, it was crticial that the military stated early on that they would not engage in mass killing of the people. But in some ways the real tipping point, when the military went from "see what happens" neutrality to deposing Mubarak, was when the workers' strikes started and kept expanding. The latter was real threat to the military (which is a major domestic economic force itselft) and other economic elites. Both the willingness of working people to suffer short term economic deprivation in support of regime change, and the economic consequences of their striking is the tipping point for both the military and the elites to agree to regime change.
Why do militaries not kill the people? At some level there has to be a seperation between the regime and their military. And also a degree of ties between the military and the people: In Egypt, most families, urban and rural, have connections (in the current or recent generation) to the military. In China, in 1989, the military (especially those elements specificially brought in from the outside to break up the demonstrations in cities) is often described as being made up of rural conscripts for whom the military was a way to get ahead, and whose background led them to be alienated from urban folk.
Also, in China, in 1989, it was when there began to be worker strikes, in industry and mining, that the regime reacted fast. It was always very telling that while many of the more well known democracy activists were just jailed, the more anonymous labor activists just got a swift bullet in the head.
Finally, let me distinquish less analogous situations: throwing off outside colonial rule, be it Britain from the 13 American Colonies in the 1770s, to the many freedom movements of the 1940s through 1960s. With the rulers being a tiny minority in-country, and having to send in troops from the outside, that is a different scenario. Similarly a regime made up of a tiny internal distinct minority oppressing a much larger majority, such as in South Africa under apartheid (and the perhaps the Allawite regime in Syria?) is a different story. So is the occupation and settlements by Israel of the immediately adjacent Palestinian people.
Finally, as we have seen with the effect of food (and other commodity) prices in the recent revolutions, it has ever been thus . Famously, the sans culottes, the working class and those on fixed income, went into French streets of the 1780s-1790s whenever there was spike in food prices. As Brecht-Weill put it in the Three Penny Opera: Food is the first things, morals follow on.
What do you think?