What will happen in Syria following IS defeats?

Civil wars that spread devastation and suffering across a whole country have no real victors. But one war in Syria - that against the Islamic State (IS) group's so-called caliphate - is well on the way to being won.

Earlier this week IS's last urban bastion in eastern Syria, Deir al-Zour, hard up against the Iraqi border, fell to Assad government forces. IS will remain in some form or another as an insurgency and source of ideological inspiration but as a territorial entity or physical caliphate, it is finished.

But what of Syria's other war, the uprising against the Assad regime and its efforts - aided by Iran and Russia - to crush the opposition?

The current situation on the ground means that forces from the above countries will be in close proximity to United States troops, who are supporting some of the anti-Assad groups.

Joshua Landis, a Syria expert and professor at Oklahoma University, summed it up in simple terms. "Assad has won the Syria war militarily," he told me. "He has defeated the original uprising or revolution. The rebel groups that remain have been pushed to the margins of Syria.

"The international community has all but abandoned them as a lost cause. The rebel militias," he argues, "still have some teeth in defence, but cannot mount a credible offensive against Assad's military."

Charles Lister, a senior fellow at the Middle East Institute, and another close watcher of Syria, has a slightly more cautious assessment. "President Assad," he notes, "sits more comfortably in Damascus than at any time since 2011."

But having said that, he argues that "it would be inaccurate to suggest Assad had won the war. He's simply avoided losing it."

"The Assad regime has a stated intent to recapture every inch of Syria. If that goal is to ever be met, we're talking years at least," he explained.

But the crucial take-away from all this is that Syria is entering a new phase of conflict. The territorial defeat of IS, says Charles Lister, "will throw an awful lot of potential sources of hostility up into the air and nobody really knows right now how they'll land".

What is emerging is a new strategic map with Syria divided into different zones: One controlled by the Assad regime (with the support of Russia and Iran), another controlled by the Syrian Democratic Forces (an amalgam of Kurdish, Arab and other groups supported by the US), and others run by various elements of the Syrian opposition, backed to varying degrees by Turkey and Jordan.

• The rise and fall of the Islamic State group

• What should happen to IS fighters in Syria and Iraq?

• Raqqa: IS 'capital' falls to US-backed Syrian forces

• How much territory have Russia and Syria recaptured from IS?

Having helped Assad restore his control over a significant part of the Syrian population, Moscow has also manoeuvred itself into holding the best cards in the putative diplomatic end-game.

As Joshua Landis told me, the Astana peace process, led by the Russians, "is the only one worth anything at the moment.

"The Geneva process, led by the US," he notes, "has been about grandstanding and sticking to talking points that no longer have any relevance on the ground, such as demanding that Assad step aside and that democratic elections be held in Syria. Everyone knows this will not happen."

With the demise of IS, Syria's future will continue to be determined by a variety of external players, fighting out their own strategic battles and seeking local advantage.

The four key actors are the US, Russia, Turkey and Iran.

• The United States

Special forces from Western countries, including the US, have supported Kurdish-Arab forces in Syria

Its initial half-hearted efforts to galvanise a democratic opposition to defeat the Syrian regime failed dramatically. Its focus has largely been on the defeat of the IS caliphate.

But now, Joshua Landis says, Washington must make a decision: "Will it stay in Northern Syria to defend the gains of the Syrian Democratic Forces that it has armed, trained and propelled to victory in Raqqa and the region north of the Euphrates River?"

The difficulty, as Charles Lister told me, is that "beyond fighting IS, it is sadly very hard to determine whether the US really has a Syria policy."

And he says that what policy there is is full of contradictions. For example, Washington continues to say Assad must leave and that his days are numbered, and yet the US has ceased all support to anyone opposed to Assad.

• Turkey

Turkish President Erdogan's main concern is with the Kurds

If US policy could be said to be in a mess, so too could that of Turkey.

Ankara's goal, says Joshua Landis, is to retrench. "It seriously overreached in Syria," he told me, "almost to the point of destabilising Turkey."

He believes that President Erdogan "must make sure that the Kurdish question in Turkey does not lurch toward civil war. He will increasingly normalise relations with Assad in order to contain the independence of Syria's Kurds." Turkish troops have moved a small way into northern Syria to achieve this goal.

Indeed, after posing as a champion of the opposition against the Assad regime, Charles Lister says, that "at times, Turkey has directly betrayed the opposition groups it had stood by for so long, merely to secure a more favourable position against the Kurdish YPG, which it views as a terrorist organisation.

• Iran

Shia militias, backed by Tehran, have played a prominent role in the campaign against IS

In backing the Assad regime (and offering significant support to the Shia-dominated government in Iraq) Tehran has had one clear goal - to secure its hegemony in the northern Middle East: the lands stretching from Lebanon through Syria and Iraq, all the way to Iran's own borders.

"This," says Joshua Landis, "is the new security architecture that Iran has fought so vigorously for and it is within its reach today. This means that Iran can counter-balance Israel. It means that Iran can establish oil pipelines running to the Mediterranean coast, trade routes, highways, and pilgrimage routes."

This, he says, means "Iran is no longer cut out of the Middle East."

And Tehran has troops to back up its position. Charles Lister notes that Iran "commands tens of thousands of Shia militiamen inside Syria, which gives Tehran more influence than any other actor, bar none."

• Russia

Russian troops have been on the ground in Syria

Russia, after Iran, is the other great winner from the Syrian conflict, reviving its role in the region, securing important military bases, and making itself a key diplomatic player.

It wants to "solve" Syria on its terms and with its favoured actors ending up the victors and it seems to be well on the way to achieving this goal.

But the growing proximity of Russian and Iranian-backed pro-regime forces and those backed by the US raises the possibility of some dangerous encounters. The US and Russia can agree on the need to defeat IS but on little else. Moscow's "side" has the military and diplomatic advantage on the ground.

Will the US seek to bolster its position in Syria, perhaps as part of a broader policy to "roll back" Iranian influence, as US conservatives are hoping? This may be easier said than done and might require many more resources and boots-on-the-ground than the Trump administration is prepared to put in harm's way.