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After months of military setbacks and heavy losses, Syrian regime troops yesterday readied for battle against probably the most formidable fighting force in Syria's war.

Their opponents were the Islamic State group [IS] besieging regime troops in the Kweres Syrian air force base in Aleppo province.

For two years, the government troops have been locked in an impasse with the IS fighters - and rebel units before them - waiting outside the perimeter of the airbase for the order to launch the final assault on Kweres.

The besieged soldiers have also had to fend off repeated and suicidal assaults from the extremist group, knowing that a failure to repel the attacks would mean certain, gruesome death.

Reports of soldiers beheading captured IS fighters have also emerged.

On Tuesday, a Syrian army column sent to relieve the surrounded troops finally managed to break through IS lines and lift the siege on Kweres.

Syrian state TV showed overjoyed troops letting off rare celebratory bursts of rifle fire into the air.

But once this relief subsides, many know that there will be plenty more casualties in future battles with IS and rebel forces.

A battle to end all battles?

Syria's war has changed massively in the two years the troops were beseiged in Kweres airbase.

The stranded soldiers must have seen the fighters around the base coming-and-going in a reflection of shifting political allegiances between rebel groups.

They were finally saved by a last minute shot in the arm for the collapsing Syrian regime, when Russia stepped in to launch air raids on IS forces near Kweres.

What might have stopped the base from falling earlier, as al-Tabqa airbase in Raqqa province did last year, with 250 captured soldiers stripped and shot dead by the IS - was the establishment of "the caliphate" in Mosul.

After IS leader Abu Bakr al-Baghdadi announced the establishment of the Islamic State group, his fighters turned their guns on their one-time rebel allies and captured opposition-held territories in Raqqa and Deir ez-Zour provinces.

Since then, most of the fights the IS have picked have been with rebel groups not regime troops.

It has kept IS militants busy over the past year fighting Islamist and Free Syrian Army forces, Kurds - as well as some regime troops and allied militias.

Russian air power has given the regime new reach, and in Aleppo the Syrian army managed to recapture from IS a vital supply artery to its forces inside the city.

However, elsewhere in the country the regime is still suffering serious losses.

In Hama and Homs, rebels have miraculously managed to stave off regime assaults - despite massive bombardments from Russian warplanes.

Jaish al-Islam also look set to take a government miltary base in Maan, Hama province.

After weeks of advances, rebel forces have successfully captured the villages and outposts that constitute the first line of defence for the strategically important Sahl al-Ghab plain.

Rumours that Syrian army manpower is seriously depleted, and that the regime has been unsuccessful in recruitment drives were confirmed this week when news emerged that Syrian men born between 1981 and 1987 were hauled off buses at checkpoints in Damascus.

They were allowed one phone call to tell their family that they had been conscriped to the Syrian army and perhaps might never return.

Manpower shortages

The Kweres offensive mirrored the regime's problems of finding new men for an army that has lost a huge amount of its men through death and desertion.

Although breaking the siege was portrayed as an achievement of the "heroic" national army, a large part of the advancing column was not drawn from regular troops. 

Much of the force was made up of Syrian and Lebanese Hizballah fighters and Iraqi-Shia militias, while the assault was reportedly directed by Iranian Quds Force commander Qasem Soleimani.

What the regime hopes to win from this much-publicised battle is the legitimacy of its rule, or at least a grudging acceptance from European and US powers.

"Clearly the offensive does give credibility to the idea of the regime 'fighting terrorism' but I see the push towards Kweres as part of a broader goal of securing and expanding control in Aleppo province that is designed to ensure future legitimacy of the regime," said Aymen J Tamimi, a Syria researcher.

"The calculation is that if it is certain the regime will not lose Aleppo, then it is more likely that outside actors might accept  Assad staying in power for an extended period of time, or something along those lines. In other words, negotiations tilting towards regime or Russian or Iranian terms."

However, Kweres is not fully secure. It now depends on a thin and vulnerable supply line, and sporadic clashes are continuing against IS diehards around the base.

The question is whether IS will redirect some of its forces engaged in fighting with rebel groups in Aleppo to the confrontation with the Syrian army at the airbase.

"The most notable fact about the Aleppo province is that all the main actors are fighting on multiple fronts: could IS decide to retreat and refocus energy on the north Aleppo countryside versus the rebels?" asks Tamimi.

"Could the regime forces and its allies decide to press further on IS, perhaps having the effect of easing the burden on north Aleppo rebels, although this would be certainly not be something that they would want? There are multiple possibilities."

No clear winner

A recent report by al-Jazeera suggests the Free Syrian Army is also suffering from desertions in Aleppo province due to low salaries.

Many of these fighters appear to be joining other rebel outfits such as al-Nusra Front instead of IS or the regime.

But the chaos created by these shifting alliances might be one reason why the opposition in Aleppo has not repeated the successes of their allies in Idlib, Hama and Homs.

The future course of the war might now hinge on how the rebels' many international backers - the US, Europe, Turkey and Gulf states - respond to the recent Russian-led peace intiatives such as the Vienna talks, the second round of which is due on 14 November.

If they opt for a peace plan that keeps Assad in charge during a transitional phase then this would be a massive blow for the rebels, and history might see Kweres as an important turning point in the war.

But if the Saudi Arabia-Iran/Russia spat continues, then more anti-tank missiles could find their way into rebel hands - and the regime could look as vulnerable as it ever has been during the four year war.