A supporter of Mir Hossein Mousavi during protests in Tehran
By Bridget Kendall
BBC diplomatic correspondent
The shadow of history hovers over Iran at the moment.
And it is not just the student protests 30 years ago, which helped bring down the Shah and which launched the 1979 Iranian Islamic Revolution, that are worth recalling.
Also relevant is what happened 20 years ago. 1989 was the start of the transformation of the Soviet Bloc, symbolised above all by the fall of Berlin Wall at the end of 1989, which paved the way for the abrupt collapse of the entire Soviet Union two years later, and the end of the Cold War.
The Tiananmen uprising is a reminder that not all student revolts succeed
And that upheaval tangentially helped inspire another collapse: the end of the apartheid regime in South Africa, another seemingly unexpected revolution that brought a fresh breeze of hope to world politics.
But let us not forget June 2009 also marks 20 years since the Tiananmen Square crackdown in Beijing, when a popular Chinese uprising failed in its bid to get a ruling Communist elite to accept greater democracy. It is a reminder that student revolts do not always succeed in their ambitions.
Reforming the system
So how do these transformations happen? What makes a top down, or bottom up revolution successful? And what are the factors that can stop a popular movement in its tracks, or mean that it will peter out without getting anywhere?
As it happens, this was the central theme this week of the discussion programme, The Forum, which I chair for BBC World Service radio.
Emeritus Professor of Politics from Oxford University, Archie Brown, (who has a new book out called The Rise and Fall of Communism), argued vehemently that in a dictatorial society - like the old USSR - you can only successfully reform the system when the change is top down and a reformer like Mikhail Gorbachev dares challenge the power structure from within.
Revolutions from below will get nowhere, he argued, because the threat of state repression or invasion will always act as a deterrent or the state will often act to crush an uprising with force. Think of Soviet tanks in Hungary in 1956 or the Soviet invasion in Czechoslovakia in 1968.
Reformer Gorbachev challenged the power structure from within
If a leader or ruling elite has the stomach to repress dissent in a brutal way, and retains control over the police, security services and other levers of power, they can totter on for years, however disastrous the economy or isolated the country. Just look at North Korea or Robert Mugabe's Zimbabwe.
But maybe in a more open society, "people power" can sway governments.
Think of the Rose Revolution in Georgia, and the Orange Revolution in Ukraine, where election results were successfully challenged and ushered in new leaders (although in both those countries, a new order has led to further political wrangling, and fresh leadership crises.)
Or think of the so called Cedar Revolution in Lebanon, which prompted the withdrawal of Syrian forces.
Whatever the complications since, in all these instances, we saw governments that were unprepared to use force against their own people to disperse demonstrators and clamp the lid down. In the end, the only alternative was to make concessions.
So what does that tell us about Iran?
In mid crisis, many questions remain open, events are still unpredictable.
President Ahmedinejad swept to power in 2005
But, leaving aside the current allegations of election fraud, it is worth noting that in some ways this is a country with a vibrant democratic tradition.
The election that first swept the reformist President Mohammad Khatami to power, and the election that delivered the shock result of the conservative President Mahmoud Ahmedinejad, were both dramatic and real - reflecting, seemingly, a genuine choice by the Iranian people.
This explains the current outrage of those Iranians who harbour suspicions of vote rigging and fear this election may have been "stolen".
But at the same time, the Iranian Islamic Republic's credentials as a democracy were always weakened by that extra tier of non-elected religious power, epitomized by the supreme leader - the ultimate spiritual authority whose word, it seemed, could not be questioned.
But that supreme authority is partly about perception, and even when the final arbiter of power in a country is non democratic, the will of people can have an impact.
I remember attending a seminar a few years back given by a retired member of Pakistan's intelligence services, the ISI, who explained the circumstances under which the armed forces in Pakistan were prepared to overturn a government and seize power.
An army coup, he argued, could happen only if the country was about to descend into political chaos and be dangerously weakened. And even then, he added, the military chiefs would carefully weigh up the public mood.
Ayatollah Khamenei will need to gauge the mood countrywide
If they sensed that the country would not support the army intervening, then the generals would hold back. Even non democratic institutions like the security forces, he explained, were sensitive to public opinion and needed to retain the peoples's trust.
So, too, with a supreme leader. He also needs to take careful note of the mood countrywide, to make sure his legitimacy is not fatally undermined. Ayatollah Khamenei must also be trying to gauge whether his authority is being weakened.
Of course this doesn't mean that any large street movement on its own can bring down a government.
As the BBC's former Tehran correspondent, Jim Muir, has pointed out, earlier student demos in 1999 and 2003 fizzled out because they were badly organized, too focused on limited issues, and because they lacked prominent leaders from inside the political elite to fight their battles behind closed doors.
People power, it seems, is all very well, but as we are apparently now seeing in Tehran, friends in high places who can harness a lowly grumble of dissent to a more lofty power struggle for control, can make all the difference.
And besides, it is not uncommon for the narrow concerns of a liberal urban intelligentsia to fail to make a case with a wider public. Look no further than the political left in Silvio Berlusconi's Italy - which is constantly bemused and dismayed at the prime minister's popular success in elections.
And look at Vladimir Putin's Russia, where until recently the government had no problem in sidelining liberal protests as marginal disruptions. In this former allegedly proletarian state, it is only when the ordinary folk - the unpaid workers, the destitute pensioners and frustrated mothers - take to the streets, that the Kremlin starts getting worried.
But there is another intriguing factor which in political upheavals worldwide in increasingly taking centre stage: the role of modern technology to bring people together, to get their message out and allow them to organise in cyberspace.
Already in Iran we are seeing attempts by the authorities to control the information space
Already in Iran we are seeing attempts by the authorities to control the information space. Foreign accreditation has been curtailed, foreign media teams are banned from covering events without express permission, in effect grounding them to their bureaux and hotel rooms. And jamming has disrupted the sending of text messages, foreign satellite TV transmissions - including from the BBC- and other electronic communications.
But even so, keeping information from a population determined to seek it out is not proving easy.
Jamming is complicated and expensive.
In the old days of the Soviet Union, it was shortwave broadcasts the regime targeted.
But as computers and satellite dishes replaced shortwave radios, the Politburo faced a dilemma.
I remember Mikhail Gorbachev's former ideology chief, a liberal reformer called Alexander Yakovlev once recalling how his bosses in the old Soviet Politburo ordered him to cost the business of jamming all satellite TV. This was the mid-1980s, in retrospect the last dying decade of the old USSR.
Mr Yakovlev said he looked into it, and reported back: radio jamming was expensive but just about doable. But when it came to TV jamming, it was just too expensive. The genie was effectively out of the bottle. The Politburo had to accept that the USSR could no longer practically enforce an isolated information space.
And if, like Iran, you have an overwhelmingly youthful population (more than 50% under the age of 25), thirsty for links with the outside world, too young to remember the point of the Islamic Revolution, and versed in the tricks of digital evasion, how do you control information?
Iran has an overwhelmingly youthful population
The tentacles of the internet are everywhere. Raw footage and almost real time images can be delivered to global sites in seconds.
Of course, the key here is what the younger generation believe in. The lesson of contemporary Russia tells a different story. There, youngsters ill versed in the history of the recent past, have bought into a government that sponsored false nostalgia about a benign and benevolent Soviet Union, and even worse, of the ruthless dictator Joseph Stalin airbrushed as a war hero...
But perhaps, in conclusion, there are two key issues to watch: whether a government is prepared to use force, and whether it can effectively control the information space.
And it could just be, for all the power of cold metal bullets, that it is the second virtual tool that is the most crucial.
In a media blackout, a regime can crack down and arrest people more easily, when it knows that the violence will not fuel the anger of protesters and/or fan the indignation of worldwide opinion.
You can listen to the BBC World Service's The Forum with Bridget Kendall
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