In the early hours of 24 July a Saudi volunteer belonging to the Islamic State of Iraq and the Levant (Isis) drove a car packed with explosives towards the perimeter wall of a base manned by 300 soldiers of the 17th Division of the Syrian army near the city of Raqqa in north-east Syria.

As the Saudi raced at high speed towards the wall he was given covering fire by a barrage of artillery shells and rockets, but he did not quite make it. His car was hit by Syrian army fire and blew up with an explosion that shook buildings miles away in Raqqa city. The plan had been for 40 Isis fighters to burst through a breach in the perimeter wall made by the suicide bomber. A further 600 Isis fighters were to follow up the first assault, if it made headway.

A second Saudi suicide bomber in a truck drove towards the base, but his explosives also detonated prematurely when hit by Syrian fire. Even so, the Syrian army detachment appears to have been too small to defend the base and 50 of them were ambushed and killed as they pulled back. A Twitter account linked to Isis later showed horrific pictures of the heads of decapitated soldiers stuck on the spikes of what looks like a gate.

It turned out that the assault on the 17th Division was not even Isis’s main assault which was directed against Regiment 121, a major Syrian army stronghold outside Hasakah City in north-east Syria. The regimental commander General Mozid Salama was reported killed and pictures posted by Isis show captured T-55 tanks, artillery pieces and multiple rocket launchers. Omar al-Shishani, a Chechen rebel commander, issued a statement saying the battle had gone on for three days, during which there were “dense missile, air, artillery, tank, machine gun and sniper fire on small mujahedin assault groups”. He added that 50 guns, including a 120mm artillery piece, and two tanks had been captured by his forces.

The fighting was among the most severe between the Syrian army and the armed opposition for a year. It put an end to a conspiracy theory that President Bashar al-Assad’s army and Isis secretly collaborated and never fought each other. The victories of Isis, which has taken over much of eastern Syria in the last three weeks, have established its position as the dominant force among the Syrian rebels. It has driven the al-Qa’ida affiliate Jabhat al-Nusra out of the oil province of Deir Ezzor and other groups are disintegrating as their fighters defect to Isis, attracted by its astonishing victories in Syria and Iraq since the fall of Mosul on 10 June.

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There is no sign that Isis is running out of steam in either the Syrian or Iraqi parts of the caliphate declared by Abu Bakr al-Baghdadi on 29 June. In both countries its fighting force is growing in numbers and effectiveness, if not in popularity. In Mosul its blowing up of the Sunni mosque above the Tomb of Jonah, as well as the destruction of at least 30 other Sunni and Shia shrines, has dismayed local inhabitants.

“Believe me the destruction of the ancient mosques and the persecution of the Mosul Christians have left everyone here helpless,” writes a Sunni woman living in Mosul. “We are very angry and bitter.” But the anger is mixed with helplessness and there is no sign of a counter-revolution by the Iraqi Sunni against Isis which is becoming militarily more powerful by the day. Arabic television stations like al-Arabiya and Al Jazeera, see hopeful signs of Isis being displaced by the Sunni tribes, neo-Baathists and ex-army officers as happened in 2006 during the American occupation. But this time around Isis is expecting a stab in the back and has taken counter measures by demanding that all swear allegiance to the caliphate and arresting those it suspects of disloyalty.

Its run of victories makes Isis difficult to displace and there is no sign of these ending. It is increasing its stranglehold on Baghdad and a government counter-attack to recapture Tikrit failed dismally. Shia volunteers who answered a call from Grand Ayatollah Ali al-Sistani to help the army are streaming home disillusioned and complaining that they suffered heavy losses when they fought and were left without food, arms and ammunition. Nouri al-Maliki, whose maladministration is considered responsible for recent disasters, is still Prime Minister. For many Shia he is the beleaguered leader of their community whom they see as betrayed by the Kurds who expanded their quasi-independent zone by 40 per cent after the fall of Mosul.

Isis has seized most of the wholly Sunni parts of Iraq outside Baghdad, where there are large Sunni enclaves, and south of the capital where there are strategically placed Sunni towns. Advances into mixed or purely Shia districts will mean harder fighting and heavy casualties. Isis, which so far has made few military mistakes, may feel it is easier to take ground in Syria, particularly north of Aleppo from which it made a tactical withdrawal earlier in the year. It may want to eliminate or bring under its sway other rebel groups so, as in Iraq, there is no opposition military force around which its enemies can rally.

Isis has been lucky in that its advances in eastern Syria have taken place as international attention is absorbed by events in Ukraine and Gaza. The Shia political leadership has taken refuge in wishful thinking that the Sunni community is open to a power-sharing deal and regional autonomy. In fact, there is no evidence that Isis or its Baathist allies want to end a war that so far they are winning. Isis might not be able to storm Baghdad by a direct assault but it could reduce it to mayhem by bombs or by blockading it. “If the fall of northern Iraq was the first act of this tragedy, then I suspect there is second act still to come,” said one Iraqi observer.