The North Atlantic Treaty Organization is a military alliance that is chiefly defensive in nature. Under Article 5 of NATO's Washington Treaty, an attack on one member of the alliance is seen as an attack on all of the others as well.

"The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all," the treaty says. Former NATO Secretary-General Anders Fogh Rasmussen put it this way: "It's all for one and one for all."

One step short of this is Article 4, which provides for consultations at the request of any of NATO's 28 member states:

"The Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened."

Though Article 4 has been invoked several times since NATO was formed in 1949, Article 5 has only been invoked once: by the United States on September 11, 2001.

This could change in light of the recent advance of "Islamic State" (IS) militants in Syria and Iraq, as they could pose a threat to NATO member state Turkey. IS fighters have been advancing closer and closer to Turkey's border in recent days as they seek to take the Syrian border town of Kobani. Should they succeed in taking the town, IS would control a long section of the Syrian side of the border with Turkey. Ankara has repeatedly threatened to invoke Article 5 if IS fighters attack its territory.

The Suleyman Shah dilemma

Due to its concerns about possible attack, Turkey has deployed thousands of soldiers and dozens of tanks in defensive positions along its border with Syria. Its NATO partners are also providing support through the military alliance's "Active Fence" mission. German Bundeswehr soldiers are among those manning Patriot surface-to-air missile defense systems to help secure Turkey's airspace.

Turkey is not only worried about its side of the border. There is also the tomb of Suleyman Shah, located on a peninsula in the Euphrates river about 30 kilometers (18.6 miles) south of Kobani. Turkey regards the enclave as its territory and it is defended by around 35 soldiers. Ankara has also threatened to invoke Article 5 if the enclave is attacked by the IS fighters who control the surrounding area.

Whether the peninsula actually qualifies as Turkish territory is a matter of dispute. The 1921 Treaty of Ankara between Turkey and France declared that the tomb remained Turkish territory despite the fact that it was located about 35 kilometers (20 miles) inside Syria. The treaty gives Turkey the right to maintain a small military presence and raise its flag in the enclave. NATO has yet to state its position on the issue. However Jens Stoltenberg, who recently took over as its secretary general, has pledged that "NATO will be there if there is any spillover, any attacks on Turkey as a consequence of the violence we see in Syria."

The buffer zone dilemma

Another possible scenario for which the legal framework is not clear is if the Turkish military were to cross the border in an effort to create a buffer zone. The parliament recently passed legislation that would allow Turkey's army to carry out operations on Iraqi and Syrian territory to do just that.

It is not clear whether Ankara could invoke Article 5 if its military were deployed on Syrian territory. The former NATO general and Bundeswehr commander Harald Kujat told the mass circulation Bild newspaper that "if Turkish troops were to conduct operations in Syria without Syria's permission and without a UN mandate and then be attacked this would never amount to a casus foederis (Article 5)."

Fighting 'IS' in the absence of Article 5

It is unclear what kind of additional military support Turkey could expect even if Article 5 were successfully invoked. Article 5 does not compel members states to take military action, instead leaving it up to each individual country to take "such action as it deems necessary."

Even in the absence of Turkey's invoking Article 5, individual NATO members are already fighting Islamic State militants, with by far its most powerful member, the United States, launching airstrikes on targets in both Iraq and Syria. Britain and Australia have joined the bombing campaign. Germany, on the other hand, is supporting the Kurdish Peshmerga forces by supplying them with weapons.