Yesterday, Turkey's parliament has backed a motion that could allow its military to enter Iraq and Syria to join the campaign against Islamic State (IS) militants. Turkey had been reluctant to join its NATO allies in a coalition against the Islamic State militants, citing worries about the safety of Turkish hostages held by the group. It reversed its decision after the hostages' release earlier this month. While Arab countries including Saudi Arabia, the United Arab Emirates, Jordan, Bahrain and Qatar have quickly joined the bombing campaign, Washington's traditional Western allies had been slow to answer the call from U.S. President Barack Obama. France was the first Western country to respond, but this week national parliaments in the United Kingdom, Belgium, the Netherlands, Denmark and Australia have approved to join the global coalition against Islamic State too.

The Islamic State (IS), formerly known as the Islamic State of Iraq and the Levant (ISIL), is a radical islamist group aiming to establish a "caliphate", a state ruled by a single political and religious leader according to Islamic law, or Sharia. IS represents a clear threat to US allies in the region and, given the significant numbers of foreign fighters in its ranks, potentially to Western countries as well. Although currently limited to Iraq and Syria, IS has promised to "break the borders" of Jordan and Lebanon and to "free Palestine". IS militants have advanced to within a few kilometres of the Kurdish town of Kobane, on the Syrian border with Turkey. The Islamic State has prompted thousands of Kurdish refugees fleeing the fighting in Syria to enter Turkey, which is already hosting more than a million Syrian refugees.

The motion of Turkey's parliament provides a legal framework for the Turkish military to launch incursions into Syria and Iraq against militants who threaten Turkey. It also allows for foreign troops to be stationed in Turkey as part of the same campaign, enabling the US and NATO-allies to use its large airbase at Incirlik in southern Turkey for air strikes against IS.

On Friday 15 August 2014, the United Nations Security Council unanimously adopted a resolution aimed at weakening militants in Iraq amid reports of Islamic State militants killing dozens of residents in besieged ethnic Yazidi areas. The Security Council deplored and condemned in the strongest terms “the terrorist acts of ISIL and its violent extremist ideology, and its continued gross, systematic and widespread abuses of human rights and violations of international humanitarian law.” The resolution, under Chapter VII of the UN charter, allows use of “all necessary measures” to “suppress the flow of foreign terrorist fighters” to Iraq and Syria, where IS has made widespread advances during which it has been accused of committing atrocities.

The US and its allies will not do the fighting on the ground. But they will provide air power and the means to ensure that the application of that air power is effective. President Obama has framed the use of Western air power very much in terms of helping Iraqi forces on the ground.

Unlike in Syria, the government in Iraq has welcomed Western intervention. Dutch Deputy Prime Minister Lodewijk Asscher said the Iraqi government's request for support gave the Netherlands adequate legal justification to take part in the military intervention. Dutch Defence Minister Jeanine Hennis-Plasschaert added that although the Dutch mission will initially be in Iraqi air space, it could be broadened to include Syria: "We are not ruling out taking part in Syria, but for now we are limiting participation to Iraq and we will follow international developments."

British Prime Minister David Cameron suggested that there would be a strong case for expanding Britain’s air campaign to Syria — but said that would require a separate debate. Labor Party leader Ed Miliband has maintained that he would like to see a United Nations resolution authorizing strikes in Syria before he will back such a move.

Although there is a rather strong moral justification for Western countries to join the Coalition against Islamic State to intervene in Syria too, the legal justification seems to be missing. The Syrian government didn't ask for any help yet. But is the war over in case the Islamic State has been destroyed in Iraq but continues to fight in Syria? What will happen to the public opinion in Western countries if the threat inside their own borders is more imminent than expected by their intelligence agencies? Worst of all, there is no timepath: will it take months, years, decades to control IS? Turkey's President Recep Tayyip Erdogan urged the West to find a long-term solution to the crises in Syria and Iraq, pointing out that dropping "tonnes of bombs" on IS militants would only provide a temporary respite...