Despite the tireless diplomatic efforts of US Secretary of State John Kerry, the US is less concerned strategically about what happens between the Assad regime and the Syrian armed opposition groups than it is in eliminating IS and the threat it poses to the government of Iraq – and the West.

The European members of the US coalition and NATO are also, of course, interested in a ceasefire as a means of slowing the asylum seeker flow into Europe, about half of which is related to the civil war in Syria.

The most effective US coalition weapon against IS has been air strikes. They are essential to support ground troops advancing without any other substantial fire support. In the past 12 months, the only advancing combatants against IS in Syria or Iraq have been Kurdish militia in Syria and Iraq, and Iraqi security forces and Shiite militias in Iraq.

Western support for each of them comes at a cost. While the Kurds are the US's most effective fighters in Iraq and Syria, Turkey is opposed to any US support for the Kurdish militia whom they see as a long-term threat to Turkish sovereignty. The Kurds are now getting to the limits of what they consider to be Kurdish areas in Iraq and Syria and do not seem motivated to advance into areas that have traditionally been Sunni areas. Some would prefer to be operating in Kurdish areas of Turkey instead.

The US and Australian-trained Iraqi security forces are not an effective fighting force because they are poorly led and lack motivation. They are Shiites like the Shiite militia and Badr Brigade that have been doing much of the effective fighting, often under Iranian military leaders, to dislodge Sunni IS from Sunni towns along the Tigris and Euphrates Rivers. But there are major social implications in having Shiite forces advance into Sunni areas where in the past the Sunnis have been exposed to Iraqi government corruption and government-tolerated Shiite death squads. In many IS occupied areas, the local Sunni population sees IS as a lesser evil than the Shiite-dominated Baghdad government.

Looking at other alternatives to use against IS, there are the Syrian armed opposition groups, but they are more interested in fighting President Bashar al-Assad's regime and each other than they are in fighting IS. Past US attempts to train them to fight IS have been expensive and spectacularly unsuccessful.

Deployment of US or coalition ground troops to fight IS – other than Special Operations Forces – is not a politically acceptable option for Washington whose forces would have to do most of the heavy lifting and suffer most of the casualties.

This effectively leaves the al-Qaeda affiliate JN which is an implacable enemy of IS. Should we be supporting it against IS?

It is useful to look at the recent history of JN and IS.

In 2011 IS (then IS in Iraq) leader Abu Bakr al-Baghdadi sent eight Syrian members under Abu Mohammad al-Golani to Syria to set up a Syrian branch. He recruited other Islamist fighters and in 2012 the branch called itself "Jabhat al-Nusra l'Ahl as-Sham" (Support Front for the People of the Sham), better known now as just JN or the Nusra Front.

Al-Golani was successful in building up a substantial force in Syria and in 2013 al-Baghdadi decided to draw it back into IS in Iraq. Al-Golani objected because he wanted JN to remain a separate Syrian group under his command. This led to a falling out between al-Baghdadi and al-Golani. Al-Golani then appealed to al-Qaeda leader Ayman al-Zawahiri, who was the mentor of both JN and IS, to intercede and protect al-Golani's claim to be the autonomous leader of the al-Qaeda affiliate in Syria.

After eight months of fruitless discussion with al-Baghdadi, al-Zawahiri lost patience and in February 2014 severed the relationship between al-Qaeda and IS. Since then the two groups, JN and IS, have been deadly enemies.

IS at about 19,000 to 25,000 fighters in Iraq and Syria is larger with better military leaders than JN (11,000 fighters in Syria). When they have met on the battlefield in Syria, particularly in the Deir al-Zor Governorate, the fight has invariably gone against JN. When its members have been captured by IS, including Australians Yusuf Ali, 22, and his wife, Amira, they have often been executed. At other times, IS local commanders have given captured JN fighters the option of joining IS instead of being executed.

JN is more moderate than IS in the sense that it does not go in for the kind of brutality practised by IS – which has always been frowned on by core al-Qaeda – nor does it attack Western-backed forces in Syria. That should make it more acceptable for Western intelligence services to covertly support JN against IS.

In the future it should be easier to roll up JN than it would be to roll up IS. Supporting an al-Qaeda affiliate would take some lateral thinking on the part of Western intelligence agencies but it does make sense if we think about using whatever means are available to eliminate the greater evil of IS in Syria. Even if JN could not prevail against IS they would wear each other down, which would be in our strategic interest.

What we should originally have done is support the Assad regime against IS, but unfortunately the West missed the opportunity to promote more moderate regime behaviour and work towards political reform in Syria. Now that Russia is supporting the regime militarily it will not be possible for coalition countries like Australia to do anything other than back the US which is locked into an anti-Assad, anti-Russian and anti-Iranian mindset – but without any coherent grand strategy for the region.

In conclusion, if we hope to eliminate IS in Syria we should think about what means might be most effective on the ground – and therefore should not exclude supporting JN as an option.

Clive Williams is an adjunct professor at the Australian Defence Force Academy and an Honorary Professor at the Australian National University's Centre for Military and Security Law.