The Nusra Front, al Qaeda's official Syria branch, is playing a long game in Syria, and will be best placed to fill the vacuum should ISIS collapse in rebel-held northeastern Syria.

The claim of responsibility by al Qaeda in the Arabian Peninsula (AQAP) for the deadly attack on the Charlie Hebdo newspaper in Paris on 7 January has temporarily shifted attention back to what for some had become the lesser of two evils in comparison to the headline grabbing Islamic extremist group, the Islamic State of Iraq and Greater Syria (ISIS).

ISIS has emerged as a rival to al Qaeda since a split in Syria in April 2013. ISIS declared a merger between it and Nusra but that claim was quickly rebuffed by Nusra leader Abu Mohammed Golani. And al Qaeda leader Ayman Zawahri ruled that the merger was a rogue move, later formally disassociating al Qaeda from ISIS and effectively sanctioning Nusra as al Qaeda's legitimate arm in Syria.

While the Nusra Front and ISIS share a common end goal of forming an Islamic state, they differ in method and scope, and, critically, in the implementation of Shariah.

How ISIS plans to build an Islamic state

ISIS has adopted a top-down approach, declaring the existence of the caliphate in June 2014 and imposing its form of governance through a combination of fear and incentive. It has wrested control and administration of a vast swathe (up to 65%) of Syrian territory across the country's north east.

Both ISIS and Nusra have capitalised on Sunni sympathies among the majority Sunni population in Syria who feel aggrieved and targeted by the ruling Alawite-dominated Assad regime and its Shiite supporters from Lebanese Hezbollah, Iranian and Iraqi militias. ISIS managed to consolidate control among a more sympathetic population in July 2014 in Iraq, where the Shiite-led government of Nouri al-Maliki had fermented sectarianism, and where ISIS's parent organisation, the Islamic State of Iraq (ISI), already had a strong presence.

Syrian Sunnis have tended to be more moderate in their interpretation of Islam and ISIS initially encountered greater resistance there to its brutal administration. Nonetheless, as conditions became more desperate in Syria, ISIS consolidated control of territory, securing oil and other funding sources. The group has provided education and medical services, bread and oil, as well as paying civil servants and fighters salaries that exceeded those provided under Assad rule, while also suppressing resistance through a brutal campaign of fear and punishment.

Interviews with civilians from Raqqa indicate many Syrians are adapting to ISIS rule and in some cases, actively support the group in the absence of any alternative. Lightening offensives in Iraq secured cash, arms and other revenue sources, allowing ISIS to gain primacy over Nusra in Syria in 2014. The slash and burn campaign has seen ISIS grow exponentially to some 50,000 fighters, also attracting members of the Nusra Front.

How Nusra plans to build an Islamic state

Nusra, on the other hand, has taken a bottom-up approach to establishing a caliphate, working to secure local support through effective administration in areas it controls, and working with other armed militias, particularly early on in the war when the Assad regime was the primary enemy. Nusra adopted methods such as suicide bombings and IEDs against the Syrian regime almost as soon as its formation was announced in January 2012, but it has been careful to avoid civilian casualties. Critically, while ISIS imposes harsh punishments for crimes including theft and adultery in areas it controls, Nusra adopts the policy that 'hadud' should not be imposed during times of war. This gradual and more lenient imposition of Islamic law has led to the perception of Syria's al Qaeda branch as the 'more moderate Islamic extremists'.

The fact that a greater proportion of Nusra fighters, unlike ISIS, are Syrian rather than foreigners has garnered a lot of local popular support for the group. As one humanitarian worker operating in Syria from southern Turkey described it: 'I support Nusra because they are Syrian and they are fighting Assad. I am a Muslim, I want an Islamic state, but not like ISIS.' When originally designated a terrorist organisation by the US in December 2012, thousands of Syrians demonstrated under the banner 'We are all Nusra'. In recent weeks, however, Nusra has stepped up more brutal punitive responses, notably in the de-facto 'emirate' it has imposed in northern Idlib, in what may be a harbinger of the style of its governance in any eventual Islamic State, or a sign the the group feel the need to compete with ISIS by mimicking its more successful tactics.

The effect of the US air campaign

It was such tactics, including the beheadings of American and British journalists and aid workers, that prompted US President Barrack Obama to announce in September an international coalition to 'degrade and ultimately destroy' ISIS, using a four-part approach: airstrikes against ISIS targets in Syria and Iraq; cutting off funding and improved intelligence to stem the flow of foreign fighters to the group; the bolstering of humanitarian assistance to civilians; and, crucially, strengthening support of partners on the ground to fight ISIS.

The airstrikes, which began in September 2014, will help erode ISIS administrative capacity, increasing the likelihood of internal division. But without a viable partner on the ground to fill the vacuum created by ISIS erosion, US success will be limited. Any partner will need to both tap into Sunni support and be armed and organised enough to administer territory. They must also be Islamic enough to gain the support of those who want a state based on Shariah, and moderate enough to encourage resistance to authoritarianism.

This is a tall order. The US and its allies have announced that a program to vet, arm and train an army of 5000 'moderate rebels' to fight ISIS is underway, but to date it looks woefully inadequate. These efforts were stepped up last week with the announcement that the US will send 400 troops to train Syrian rebels to fight ISIS in Iraq. Illustrating the challenges and urgency of the task, the two opposition brigades picked as the best option for a proxy force to fight against both ISIS and the regime — the Hazm Movement and the Syrian Revolutionary Front — were disastrously routed by Nusra in Idlib in November. Media reports said many fighters defected to Nusra.

While the regime and the Americans have been distracted by fighting in the north around Raqqa and Aleppo, Nusra has continued to consolidate its support base in the central provinces of Homs and Hama, also extending control in Idlib. Nusra is now the dominant force in southern Syria, along the border with Jordan and Israel along the Golan, securing not only territory but local support through functional local administration. The Israelis have maintained an uneasy truce with Nusra on the border, but it is unlikely to hold.

For now, Nusra will avoid further armed confrontation with ISIS, biding its time and positioning itself as the prime beneficiary should airstrikes and local resistance contribute to a crumbling of ISIS administration.

Where Nusra faces a significant disadvantage is in public affairs and international recruitment. ISIS has proven adept in this field, using highly sophisticated recruitment and propaganda campaigns to attract foreign fighters, tapping into Sunni marginalisation and presenting itself as the pre-eminent anti-Western jihadist terror outfit. In this context, the Paris attacks, and al Qaeda's quick claim of responsibility, could be seen as an attempt by al Qaeda to re-affirm its anti-Western credentials and tap into the ready number of recruits spawning in Europe and abroad.

The prospect of Nusra replacing ISIS as the only viable alternative with administrative capacity over large swaths of Syrian territory presents a policy nightmare for the US. While the group may appear more moderate, al Qaeda is al Qaeda. Nusra may have taken the strategic decision to limit its policy of expansion now, but the time will come when the establishment of the caliphate is prioritised, and it will not adhere to the democratic and pluralistic Syria the US hopes for.