Amid the turmoil of the events in Egypt, one question is troubling policy-makers from Washington to Europe. Is Egypt going to follow the path of Iran in 1979, from US-backed dictatorship to revolutionary Islamic government? The question is trumpeted by the Israelis, and picked up by their echo chambers in the US.
"We've seen this movie before," says Mort Zuckerman, the US billionaire publisher, who is in London to rally the Europeans against the idea of free elections in Egypt. "The last thing we need is a radical Islamist movement like the Muslim Brotherhood taking over in Egypt."
The Obama administration is clearly terrified of the charge that it is handing Egypt to the Islamists. The debacle in Iran destroyed Jimmy Carter's fumbling presidency. Whatever the complexities of Egyptian politics, nothing will be easier for Mr Obama's enemies in Washington than to cast his administration as Carter II.
True, there are some small similarities between Iran in the last days of the Shah and Egypt now. There have been mass protests in both cases, led by urban youth, often educated and frustrated. In both cases, there has been a powerful religious force in the background, the exiled Ayatollah Khomeini in the case of Iran, the Muslim Brotherhood in Egypt.
There is also a broader truism: those who start revolutions rarely end up reaping the rewards. This was the case in Russia in 1917, where the bourgeois parties that took over from the Tsar were eaten up by Lenin's small but ruthless Bolsheviks. In Iran, it was the forces of the left who delivered the death blow to the Shah's regime. The mullahs hanged them all, and took power for themselves.
To borrow Mr Zuckerman's image, the remake of a great movie is often a flop. The differences between the two situations are greater than the similarities.
In 1979 Ayatollah Khomeini was a mystery to the West. When he was exiled to a quiet suburb of Paris in 1978, I recall that the press treated him as a weird time traveller from the past, a personage too far outside their liberal universe to comprehend. No one studied his works, which were banned under the Shah, or had much understanding of his doctrine, wilayat al faqih, under which he would effectively become the interpreter of God's will.
But he was surrounded by young, highly educated Iranians who put the message across to a rather confused media that Khomeini was the man to stabilise Iran, and that he would not take any position in government. So he came to be seen as an unsettling but just tolerable figurehead who would be packed off to his dusty books in Qom.
If the real Khomeini was a surprise, the Muslim Brotherhood is an open book, part of Egyptian life since the 1940s. One thing that it does not have is a charismatic leader waiting in the wings. Nor is there any independent Muslim hierarchy in Egypt such as exists in Iran to be mobilised in favour of a Muslim Brotherhood takeover.
The Brotherhood has been conspicuous by its absence in the street demonstrations. No one has seen a single banner saying "Islam is the solution". No doubt the Brotherhood leadership is wary of giving the outgoing regime and the Americans an excuse to claim that the protests are manipulated by hostile political forces.
But some analysts believe the Muslim Brotherhood is less powerful than it seems, and without the repression it has suffered for decades, the Brotherhood would lose its mass appeal. This is a gamble, of course. The power of underground organisations is by definition hard to assess.
Geneive Abdo, a former journalist who has worked in both Iran and Egypt and written books on both countries, believes that the Brotherhood has missed its chance to seize power, though clearly it will play a role in the post-Mubarak Egypt.
"The movement seems to have missed the historical moment when it could have captured a powerful place in the corridors of power. That window began closing in 2005, after the Brotherhood captured 88 seats in the Egyptian parliament only to be targeted aggressively and largely suppressed by Mubarak's security services ever since."
During the past five years, she writes, a new Egyptian generation has arisen that is "more worldly and not loyal to any particular organisation or movement".
Egypt is not Iran. It cannot live on oil revenues and slogans. It has to make its way in the world through a real economy - trade, tourism and investment - without which it will starve.
Nor is 2011 the same as 1979. Despite the crowing from Tehran over the end of the Mubarak era, the model to follow these days is from Turkey, rather than the peculiarly Shia idea of an Islamic republic governed by the religious hierarchy. If the Brotherhood could be incorporated into a democratic system, as its members say they want, it would mark a historic compromise for the Arab world.
For the moment, comparisons with Iran look facile, and probably dangerous.
Scare stories about the Iran option seem more likely to produce the opposite effect. If this idea becomes dominant in US policy-making, then rather than a transfer of power to a Brotherhood-influenced government, power is likely to rest with the military.
The army has so far played its cards with great skill. The protesters are beholden to the army for their continued presence in Tahrir Square. It is not difficult to predict a result under which the military rules through a continuing state of emergency, while the leaders of the protest movement are quietly cowed.
The supporters of the Iran comparison would see that as a victory for stability. But it would be a wasted opportunity.