The arrest of Hosni Mubarak, the former Egyptian president, and his sons, Alaa and Gamal, pushes the Arab revolutions into uncharted waters. In the past it has usually been expedient to let deposed leaders go into gilded exile, as was the case with King Farouk in 1952.
This has two advantages: it gets the old leader out of the way and allows the new regime to make a clean start. But times have changed. Arab populations have, quite rightly, higher expectations of justice and transparency these days.
They are also incensed by reports of billions of dollars stolen by the deposed first families. There is desire to get this money back, and these days there are mechanisms to find the missing millions, even though estimates of the Mubarak family's wealth of up to $70 billion are probably grossly exaggerated.
The demonstrators who have been pressing for the arrest of the former president can hardly believe their success. As one tweeted on Wednesday night, "Good night Egypt, today we sleep happy and then wake up to think about our other duties."
How far the process will go is still unclear. It is widely assumed that the army, which is running the country by decree until elections can be held, did not want to see its patron dragged through the courts. But the generals felt they had to throw a bone to the demonstrators who have been demanding the arrest of the former president for alleged abuse of power and the deaths of the 800 people who were killed as he defended his regime.
It is not clear whether Mr Mubarak will ever go on trial. In the last pictures taken of him in office, Mr Mubarak looks like a sick and slightly bewildered old man, and he is said to have heart trouble. It is quite possible that, given the slow pace of Egyptian justice, he will never face trial.
Whatever happens, a precedent has been set. The time has passed when autocrats can leave office with "dignity and prestige", as the embattled president of Yemen, Ali Abdullah Saleh, has pleaded to be allowed to do. The perverse effect of the heightened demand for justice is that presidents will be less likely to depart, raising the possibility of civil war.
Without wishing to go back to the times of sybaritic exile of King Farouk, the demands of justice must be tempered with expediency. The protesters in Cairo will regard this advice as a betrayal. If Egypt is going to become a law-based society where leaders are held accountable to the people, they say, then now is the time to start, by making an example of the Mubarak family.
Not surprisingly, they will reject any advice from abroad: since George W Bush and Tony Blair will not face any trial for invading Iraq, what right have these countries to offer advice on good governance?
A good starting point should be the South African example, where truth and reconciliation were put ahead of punishing the guilty for the crimes of apartheid. The principle of restorative justice is that truth, not vengeance, will set you free. Such a process in Egypt will have to be accompanied by the regime's cronies being stripped of their unfairly gained assets.
The conduct of this process will be a test of Egypt's governing structures. They must avoid repeating the disastrous experience of the post-Soviet government in Russia which, when facing bankruptcy, handed valuable assets to a new bunch of sharp-elbowed cronies. This poisoned politics for two decades.
The South African experience was based on a lengthy process during which the old regime recognised its days were numbered, and crucially on the appearance of Nelson Mandela, a unique politician who inspired trust across the racial divide. We do not see any Mandelas in Egypt today, but then the need is not so great: Egypt does not suffer from any great racial or ethnic divide and its geography is sanctioned by millennia.
But if we look at the members of the Arab League today, we see an alarming number of countries that are divided or ungovernable, with more joining the queue. Tunisia and Egypt will survive as states whatever path their revolutions take, but the same is not true of others.
Libya seems destined for partition between east and west. In Yemen, which barely held together in the good times, separatist militias are gaining ground as President Saleh calls troops back to the capital to defend his regime.
Somalia and the Palestinian Authority are already split; Sudan is on the way to splitting, with the south about to form an independent state, while Iraq is informally divided into Arab and Kurdish parts. Lebanon staggers on, always on the brink.
The big question is what is to happen with Syria. In the early 1950s, after a series of military coups and failed episodes of civilian government, Syria collapsed as a state. Members of parliament voted according to who paid the highest bribes. The officer class followed suit, hustling for subventions from Iraq, Egypt or Saudi Arabia. By 1958 Syria had lost its identity as an independent state and collapsed into a hasty union with Egypt.
The memory of this time still haunts Syria. The fear is that democracy will unleash a free-for-all among Syria's many sects and regional interests, fuelled by outside interference. This fear still provides a bedrock of backing for the Baath Party which, for all its failures in economic and social development, has kept a lid on the demons that destroyed the country in the 1950s.
The task for Egypt is to make a successful transition, balancing the needs of justice with the requirement for reconciliation and economic growth. If that is done, and it may take years, then the sometimes justified fear of political change in the Arab world may be finally allayed.