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Greater US and NATO intervention against the Islamic State is not a perfect solution to the crisis, but it is the best of a poor set of policy options, write George Milad and

The Islamic State (IS) is the greatest threat to international peace and security since Al Qaeda was at its deadly peak. IS's brutal tactics of mass murder, beheadings, floggings and enslaving young women and girls has even been denounced by Ayman Zawahiri, Osama bin Laden's replacement and current leader of Al Qaeda.

IS now occupies about a third of Iraq. The area under its control closely overlaps with the area known as the "Sunni Triangle" during the US occupation. It also controls a large part of eastern and northern Syria.

This has effectively merged the two neighbouring civil wars into one vast Middle Eastern conflict.

This week, British prime minister David Cameron declared that he believed the West was now engaged in a "generational struggle" against IS, which posed a "clear danger to Europe and to our security". Similar sentiments have been heard from Washington, to Berlin, through to Canberra. Indeed, Cameron's view has emerged as the consensus position in the West.

Over its short life, IS has accumulated many enemies. In addition to Western governments, IS is now actively opposed by most Middle Eastern governments. IS's aim of creating an Islamic caliphate is a direct political challenge to the governments of the Middle East. So there is no shortage of possible actors who could fight IS.

The question is which one ought to.

The Shiah-dominated Iraqi military and associated militias, such as the formidable Badr Brigades and Asaib Ahl al-Haq, could unite and attempt to drive IS out of Iraq. However, this would pitch a Shiah army against a Sunni militia. Such a confrontation would give Sunnis in the region, and from further abroad, an even better reason to flock to IS's banner.

Young men from around the world would enter Iraq with the aim of defending the caliphate and preventing the perceived spread of "Shi'ism". A direct Shiah-Sunni confrontation will only deepen and widen IS's support base.

Since the collapse of Syria and Iraq into civil war, the power and influence of Iran has grown dramatically. Tehran views IS as a serious political, security and religious threat and possesses the military power to potentially confront and defeat the militia group either directly or through intermediaries, such as Hezbollah.

Nevertheless, this would be undesirable. In addition to Iran's intervention casting the conflict as a Shiah-Sunni struggle, the Iran option would result in its sphere of influence spreading from Afghanistan to Lebanon — a prospect that neither the United States, Turkey nor Israel would welcome.

The United States and European countries have shown interest in supplying weapons to the Kurds in order for its Peshmerga militias to drive IS from northern Iraq. Yet, there are limits to the Kurdish option.

First, the Kurds are too smart to get in the middle of a Shiah-Sunni Middle East-wide civil war. They will happily fight to reclaim the territory they have lost to IS, but are unlikely to drive further into Iraq or Syria. Instead, they will use the new weapons to strengthen their borders and take further political and strategic steps towards establishing an independent Kurdistan.

Next, the Assad regime in Syria could potentially go on the offensive against IS. At the moment, however, the Syrian armed opposition that not only includes IS but also the Al Qaeda affiliated al-Nusra, the Western supported Syrian National Coalition and the Kurdish militias are spending as much time fighting each other as the Assad regime. It makes sense for Damascus to save itself and allow the rival Syrian opposition groups to fight among themselves.

This leaves the United States and NATO. Recently we witnessed how effective US airpower can be against IS. With only a few dozen missions conducted by carrier-based fighters and armed-drones, the US military was able to weaken IS's siege of Mount Sinjar and permit the Kurds to recapture Mosul Dam.

The United States and NATO have made it clear that they will not intervene in Iraq with significant ground forces. However, in this instance, Western airpower could change the tide of the conflict. IS is currently advancing in civilian vehicles mounted with heavy weapons and old armoured vehicles that it has captured from the Syrian and Iraqi government forces. These are easy prey for American airpower.

A combination of airstrikes and special forces units on the ground to direct the strikes and coordinate the various Iraqi government and anti-IS militia groups can defeat IS. All this while the US takes the collective blame and, thus, avoids the worst aspects of an inter-sectarian Middle Eastern war.

Greater US and NATO intervention is not a perfect solution to this humanitarian and strategic catastrophe, but it is the best of a poor set of policy options.

This article was originally published at ABC The Drum