For all the excitement about the twilight of the dictators, only two ― Hosni Mubarak of Egypt and Zine El Abidine Ben Ali of Tunisia ― have been officially knocked over since the start of the so-called Arab Spring six months ago. It isn’t even clear whether that count will reach three.

Yemeni strongman Ali Abdullah Saleh is in neighboring Saudi Arabia for medical treatment after a bomb in his own presidential palace burned him badly and sent shards of a carved wooden prayer-niche into his body.

The Saudis have reportedly been angry at Saleh for months, since he has rebuffed their efforts to negotiate his gentle ouster. So it is probable his hosts won’t let him return home, even if he recovers enough to try and rule. But even this result isn’t absolutely certain.

The Saudis dislike disorder even more than they hate being disobeyed. If it looks like no one but Saleh can manage to keep Yemen from becoming a failed state and an even better incubator for al-Qaida in the Arabian Peninsula, then the Saudis may take steps to restore him to power.

And therein lies a tale: No one knows whether what follows the Arab dictatorships will manage to govern the unruly countries that are in the midst of unrest or civil war. Tunisia and Egypt are relatively homogeneous (though the Coptic minority in Egypt has been badly buffeted in the uncertain transitional period).

But Yemen, Syria, Libya and others are very much like Iraq: powder kegs of potential violence and divided by sect, ethnicity, tribe or some combination. The tyrants, through their secret police, kept the peace. The trouble with the Arab Spring is the Arab Fall.

If all the change afoot in the Arab world were the product of solid middle-class protesters demanding democracy and then organizing it ― a vision sometimes hinted at in the U.S. news media, not to mention al-Jazeera ― then there would be no problem at all. The truth, though, is that not only are the protesters an unknown quantity, they didn’t even bring down the regimes in Tunisia and Egypt on their own.

In both cases, it was the army that removed the dictator from office, after judging that the military’s interests would be better served by siding with “the people” than by shooting them. Events in Egypt have borne out the view that the army was prepared to negotiate shared power with whoever will be elected ― which will probably be a government dominated, though not controlled, by Islamists affiliated with the Muslim Brotherhood. In Tunisia, too, the caretaker army will make sure its power is preserved when a government ― again probably with major Islamist representation ― is eventually chosen.

Elsewhere, public protests have unleashed forces that are very different from Poland’s Solidarity or Eastern Europe’s post-1989 models of peaceful middle-class revolution. In Yemen, the violence has been between Saleh’s government and its clan rivals. At least one southern city is in outright rebellion. From the time the British left in 1967 to 1990, the country was split in two ― a situation that could conceivably recur.

One consolation is that, outside the major cities, the Yemeni government’s writ has never run very far. So in the rural and desert areas, a failed state wouldn’t look very different from what presently exists.

In Libya, public protests that began in January emboldened eastern tribes that had long been neglected by Muammar Gadhafi’s government, which is dominated by their western rivals. They took up arms, though very weakly. When France and the U.K., with the U.S. in tow, intervened ― for reasons so surprising they are best saved for a future column ― the tribes found themselves with a motive to continue a civil war that would otherwise have been a very short rebellion.

Now the greatest danger is that, if and when Gadhafi is killed or flees, the political and public infrastructure of the country will be so badly damaged that no one will be able to put it back together. In addition to the warring Arab factions, there are also Berber tribes (the preferred term today is Amazigh) that have their own language, ethnicity and interests.

Sound familiar? In Baghdad in the spring of 2003, days after the looting ended, with ministries in ruins and garbage gathering on the streets, an Iraqi in a poor Shiite neighborhood asked me, “Who is the government?” There was no good answer ― nor would there be for several years. The destruction of a state is infinitely easier than its reconstruction. The longer it takes to remove Gadhafi, the more the Libyan state is degraded, and the greater the probability that Tripoli will become Baghdad-sur-Mer.

Then there is Syria, where the protests have been brave, broad and sustained ― yet have so far failed to penetrate the main middle-class enclaves of Damascus and Aleppo. The Bashar al-Assad regime has killed hundreds of protesters and arrested ― and doubtless tortured ― many more.

The protests have, perhaps inevitably, begun to reflect the sectarian difference between the Sunni majority and the Alawite regime. The Alawites, whose religion is a kind of dissident Shiite sect, have historically made common cause with Christians and Druze (themselves religious sectaries). News reports have protesters chanting “Christians to Beirut, Alawites to the grave.”

The Assad regime is in many ways captive to Iran, and has facilitated the destructive rise of Lebanese Hezbollah. Its demise, should that occur, would in any ordinary universe be cause for unmitigated celebration. But in its wake may come ― who knows what?

The best-case scenario would be a democratic accommodation between elements of the military and Sunni Islamists ― the present and likely future arrangement in Tunisia and Egypt. But Syria is also capable of collapsing into all-out civil war. The Alawites, like Saddam Hussein’s Baathists in Iraq, have nowhere else to go, and little reason to expect future good treatment from the people they have spent decades oppressing.

The rise of democratic aspirations in the Arabic-speaking world is inspiring. Muslim democrats are going to get the chance to succeed or fail, and they are most likely to copy the Turkish model of moderation and liberal rights, not the Iranian one of religious autocracy.

But democratic transition is almost impossible when a state is weak or failing. Internal divisions make the challenge even harder. Fear of failed nation-building will make external aid scarce. In retrospect, the successes of Eastern European democratization were a near-miracle. The rise of the post-dictatorial Arab world may take an actual one. 

By Noah Feldman

Noah Feldman, a law professor at Harvard, is a Bloomberg View columnist. The opinions expressed are his own. ― Ed.