After the ignominious fall of Presidents Ben Ali in Tunisia and Hosni Mubarak of Egypt, it was widely noted that these events disproved the 'Arab exception' - the belief, held in the face of mountains of contradictory evidence, that Arabs did not want democracy.
But the overwhelming focus on the "demand-side" of an uprising lost sight of something more important: that the "market for revolution" cannot clear if the army is both able and willing to use overwhelming force. In other words, armies control the "supply-side" of revolution.
History is replete with abortive awakenings, in which the supply constraint has choked off change: Europe 1848, Hungary 1956, Prague 1968, Beijing 1989, and - what may turn into the tragic footnote to the Arab Spring - Bahrain 2011. Syria could be appended to that list in short order.
This is about much more than raw coercive capacity. Both South Korea's army in 1987 and Egypt's this year could have put up stiff resistance to the movements that swept away incumbent dictators. Why did they hold their fire?
The answer lies in civil-military dynamics. Armies that have their own identity, that possess a corporate existence separate from their political masters, often choose to manage political transition rather than simply squelch it. They see a future beyond the regime.
In Turkey, Pakistan, and now Egypt, the army has judged that it can enjoy its economic and political privileges by controlling the scope and direction of change.
In fact, outright repression would tarnish the invaluable national credentials each institution enjoys, largely as a result of its own myth-making and manipulation.
Egypt's army did try to dislodge the masses in Tahrir Square. But when it realised it could not do so without enormous bloodshed that would also wash away the institution's veneer, it shied away.
On the other hand, armies that are little more than outgrowths of an autocratic regime know that they have no institutional future if protesters get their way.
In Syria, 70% of career soldiers and 80% of officers are drawn from the ruling minority Alawite clan. Those units employed in crackdowns and massacres, such as the Fourth Division controlled by President Bashar al-Assad's brother, are all-Alawite, as are key intelligence organisations and militias.
These armies - like Bahrain's security forces and Libya's elite brigades - may be disciplined and cohesive, but they are not professional. Since their fate is bound up with that of the regimes, they have little compunction in unleashing violence.
It is this distinction, between independent and servile armies, that is one of the most important parameters in determining the trajectory of an uprising.
An independent, professional army, no matter how powerful, will have appealing alternatives to bloodshed. That doesn't guarantee a democratic revolution - see the sorry paths of 1980s Turkey or 1990s Pakistan - but it does enable a change.
Armies with a distinct corporate identity can produce dangerous Praetorian states - where the military exercises undue influence over the political regime - but they can also enable peaceful transitions.
For those armies that opt for violence, capacity does matter. The archetypal case remains Tiananmen Square. Even though 3,500 PLA officers disobeyed orders in 1989, this was only a fraction of the overall total used in the crackdown.
It helped that the PLA's 27th Army was at the forefront; this unit's troops were from northern Shaanxi Province, speaking a different dialect to the student protesters.
Bahrain learnt this lesson well, and spent years importing Sunni mercenaries from Pakistan. They have proved to be ready to fire on Shia protesters.
The Iraqi army, a mostly Sunni force, had similarly little compunction about brutally putting down the 1991 uprising in Shia and Kurdish parts of the country.
In Syria, the Assad regime cynically uses Christian and Druze troops against Sunni targets.
The UAE has gone a step further, reportedly hiring Blackwater founder Erik Prince to help establish an all-mercenary force of about 800 foreign fighters.
Ethnic difference matters.
Nonetheless, most armies do not enjoy the Tiananmen option. A weak army, or one where only select units are equipped and led by regime loyalists, will quickly disintegrate into patchwork militias.
In Libya, eastern units peeled away from Gaddafi at the outset of the conflict, sowing the seeds of a civil war. What could have been another Hama - the 1982 massacre perpetrated by an earlier incarnation of the Assad regime in Syria - became something much less simple.
All of this is not to neglect the demand-side of revolution. After all, Saudi Arabia and Qatar have all been largely quiet, and not just because of fear.
But with the understandable romanticisation of raw protest, we lose sight of the fact that massed crowds are but one ingredient of successful regime change.
For those peering into these restive states, this is a reminder that capacity-building in the absence of professionalisation simply produces more efficient slaughter.
Britain trained and equipped some of the Libyan special forces who inflicted such horrors on cities like Misrata. Western states continue to train Saudi forces, and this may well have much the same effect.
For those that find this improbable, consider that six months ago Bahrain was considered a humane and liberalising country whose parliamentary institutions obviated the need for a crackdown.
The more immediate lessons may be these: parts of Yemen's splintered forces have proven flexible enough to manoeuvre away from the Saleh regime, and might yet fall in line behind a transitional government that emerges.
Syria's sectarian army, on the other hand, will not go down without a fierce fight - one that they have an excellent chance of winning.
Shashank Joshi is an Associate Fellow at the Royal United Services Institute (RUSI), a defence think-tank in London, and a doctoral student of international relations at Harvard University.
In the wave of dissent sweeping over the Arab world, an old lesson is being re-learnt: that armies are the key to unlocking a revolution's potential.