Invincible?

Is ISIL invincible?

The territory IS controls has shrunk in the two years since a US-led multinational coalition launched an air campaign on its positions, first in Iraq and then Syria.

The coalition has managed to push IS out of the Iraqi cities of Tikrit and Ramadi, as well as an ever-increasing stretch of Syrian-Turkish borderland.

Enemies of the "caliphate", backed by (mostly) US fighter jets, are now bivouacked 50km (30 miles) from the IS "capital" of Raqqa, in northern Syria.

Yet IS' hold on its most valuable strategic terrain, the areas seized either in or before 2014, is still uncontested.

It is entrenched in Mosul and Raqqa and the Sunni Arab tribal heartland of the Euphrates river valley, which stretches from eastern Syria to western Iraq. While it has lost much of the northern Syrian borderland, IS has also expanded into areas hitherto resistant to it, such as the Damascus suburbs and parts of the Aleppo countryside.

The group's ability to acquire war spoils in the form of Russian and American tanks, recoilless rifles, rocket-launchers, light arms and ammunition was seriously diminished after it became harder to swiftly seize new territory.

When the campaign started in 2014, the US Central Intelligence Agency (CIA) said IS could muster between 20,000 and 31,500 fighters, up from an initial estimate of 10,000. Despite over a year of air strikes, in February this year the White House said IS still had up to an estimated 25,000 fighters, mainly foreigners, from more than 90 countries.

Despite its fearsome reputation and initial shock grab of territory, IS is not invincible on the battlefield.

The bulk of its rank-and-file infantry, particularly foreign, do not fight well and are often dispatched by their native commanders as cannon fodder. IS' most capable soldiers, its "special forces", have typically been Iraqi and Syrian militants with long experience of fighting in those countries, and graduates of other insurgencies, notably the Libyan-heavy al-Battar Brigade or the various units of militants from Chechnya or elsewhere in the Caucasus.

The ex-Baathists shape the group's jihadist ideology, handle its security (weeding out infiltrators or possible putschists) and military operations, and ensure its resilience. They represent a battle-hardened and state-educated core that would likely endure (as they have done through US occupation and a decade of war) even if the organisation's middle and lower cadres are decimated.