History lessons for Egypt's protesters
In every revolution, popular or otherwise, there comes a critical moment - a tipping point - at which the future is decided.
Suddenly there is an answer to the basic question: Are the protesters too strong for the power structure or can the country's leaders face them down?
In Iran, in 1978-79, the Shah resisted the demonstrators in the streets and ordered his soldiers to shoot them for several months until his will to continue gave way and he escaped.
In China's Tiananmen Square in 1989, crowds a million strong gathered - not just students but sometimes judges, senior policemen, politicians as well - but Deng Xiaoping refused to go and eventually found a general who was prepared to shoot the demonstrators down.
All popular revolutions share certain basic similarities.
The vast crowds, often gathering for the first time, believe that they are bound to win because there are so many of them and their determination is so great.
But if the political structure refuses to take the hint and keeps the support of the army and the secret police then it can survive.
It all depends on how strong and resilient the structure of government is.
In the revolutions of 1989-90 in eastern Europe the communist autocracies which had seemed so fierce, so well-based, were shown to be brittle and wafer-thin.
In Russia in 1991 the demonstrators who brought down Marxism and Leninism were few in number and nervous of the government reaction, but the Soviet government was even more feeble and collapsed without a fight.
In Tunisia three weeks ago, President Ben Ali decided right from the start that the game was up. He packed his convertible assets and took the plane to exile.
President Mubarak of Egypt is made of sterner stuff. He does not care how many demonstrators are shouting for his downfall in Tahrir Square.
But he has started to make concessions.
Having never had a deputy or a successor throughout his whole 30 years in power, he made the intelligence chief, Omar Suleiman, his vice-president and presumed successor last week.
Gen Suleiman and his colleagues know that their position is just as dangerous as the president's. They must now realise that the only way of calming the country is for the president to go.
That could just possibly be their blueprint for survival and it would run like this:
Firstly, an announcement from President Mubarak that he will not stand in the elections scheduled for next November and will therefore be resigning.
Secondly, negotiations with the various democratic parties here would take place, satisfying the outside world's demand for proper political change in Egypt - the "orderly transition of power" that the US has pointedly talked about.
The only trouble is no-one has told the crowds in Tahrir Square about this.
Their slogan is "Mubarak out now" not "Mubarak out with honour in a few months and the continuation of his system slightly improved".
They want him overthrown and put on trial.
Everyone here, after all, assumes it was he who gave the order to shoot down the demonstrators last Friday.
There is a real demand for revenge, and people here blame the president for everything from the worrying rise in prices to the habitual brutality of the police.
So, will the anger of the crowds make Egypt ungovernable to the point where the entire Mubarak power structure will collapse?
Or will the crowds gradually seep away over the next few weeks after the exciting experience of expressing themselves openly for the first time?
The demonstrators won in Iran because the crowd's anger was reignited every time the army fired on them at each 40-day celebration of the previous lot of deaths.
The demonstrators lost in China when Deng Xiaoping refused to do the expected thing and restored order at the point of a gun, though the Tiananmen option has been explicitly ruled out in Egypt by the armed forces themselves.
Then there is the eastern European model, when the crowds kept on demonstrating until the various completely unrepresentative governments simply caved in.