By 3pm last Friday afternoon, the protesters in central Cairo were chanting: Where is the army? Come and see what the police are doing to us. We want the army. And that is the main question, really. Where is the Egyptian army in all this?
Like armies everywhere, even in dictatorships, the Egyptian army does not like to use violence against its own people. It would much rather leave that sort of thing to the police, who are generally quite willing to do it.
But in Alexandria, by mid-afternoon on Friday, the police had stopped fighting the protesters and started talking to them. This is how regimes end.
First of all the police realise that they face a genuine popular movement involving all classes and all walks of life, rather than the extremist agitators that the regime's propaganda says they are fighting.
They realise that it would be wrong - and also very unwise - to go on bashing heads in the service of a regime that is likely to disappear quite soon.
Best change sides before it is too late.
Then the army, seeing that the game is up, tells the dictator that it is time to get on the plane and go abroad to live with his money.
Egypt's ruler, Hosni Mubarak, was a general before he became president, and he has always made sure that the military were at the head of the queue for money and privileges, but there is no gratitude in politics. They won't want to go down with him.
All this could happen quite fast, or it could spread out over the next several weeks, but it is probably going to happen. Even autocratic and repressive regimes must have some sort of popular consent, because you cannot hire enough police to compel everybody to obey.
They extort that consent through fear - the ordinary citizens' fear of losing their jobs, their freedom, even their lives. So when people lose their fear, the regime is toast.
So what happens once Mubarak leaves? Nobody knows, because nobody is in charge of this revolution.
The first people out in the streets were young university graduates who face a lifetime of unemployment. Only days later, however, the demonstrations have swelled to include people of every social class and walk of life. They have no programme, just a conviction that it is high time for a change.
Washington and the other Western capitals that supported Mubarak for the past three decades are praying that the revolution will choose Mohamed ElBaradei, former head of the International Atomic Energy Agency, as its leader. He flew back into Egypt last Thursday, and the regime even takes him seriously enough to put him under house arrest. But he is probably not the Chosen One.
Since it would be the army that finally tells Mubarak to leave, the military would dominate the interim regime. They would not want to put yet another general out front, so they might decide that ElBaradei is the right candidate for interim leader, precisely because he has no independent power base.
But there would then have to be elections, and ElBaradei wouldn't stand a chance.
The likely winner of a genuinely free Egyptian election, according to most opinion polls, would be the Muslim Brotherhood. The Brothers are not particularly radical as Islamists go, but the first thing they have promised to do if they win power is to hold a referendum on Egypt's peace treaty with Israel. And most Egyptians, according to the same polls, would vote to cancel it.
That would end the flow of official US aid and private foreign investment that currently keeps the Egyptian economy more or less afloat, even though it would probably not lead to an actual war. And there is no reason to believe that an Islamic government could make the Egyptian economy grow any faster, although it would distribute the poverty more fairly.
These longer-term considerations, however, will have no impact on the events of the next few weeks, when Egypt's example may ignite similar revolts against decrepit regimes elsewhere in the Arab world - or not, as the case may be. But it's not just Tunisia any more. Egypt is the biggest Arab country by far, and culturally the most influential. What happens there really matters.