It is a conduit for money, weapons, oil and fighters – and it rarely discriminates.
Whether you're a moderate rebel or an Islamic State jihadist, Turkey's 900-kilometre border with Syria has long been a vital supply route for those fighting on the front lines of Syria's brutal civil war. More than 1.5 million refugees have also fled across this border.
But Turkey's hope that any fighter – no matter how extreme – would help opposition rebels bring down the regime of Syrian President Bashar al-Assad has proven unfounded.
Instead, Assad remains in power, while extremist groups such as the Islamic State and al-Qaeda backed Jabhat al-Nusra have gained significant ground at the expense of what's left of the Free Syrian Army and the other groups fighting to overthrow the regime.
And all the while, the terror these extremist groups practise is creeping ever closer towards Turkey.
It was through this porous border that 26-year-old Hayat Boumeddiene crossed into Syria on January 8 and seemingly disappeared into the freezing maelstrom of the country's four-year war.
Her boyfriend, Amedy Coulibaly, the gunman responsible for last week's massacre in a Paris supermarket, has claimed to be part of the Islamic State, so there has been speculation she may have headed towards the Syrian city IS treats as its centre of government: Raqqa.
Boumeddiene entered Syria as thousands of foreign fighters and activists have before her – she flew into Istanbul and either drove or caught a bus on the 1000-plus kilometre journey to south-eastern Turkey. Her mobile phone was last tracked near the town of Akcakale, security officials confirmed.
Two nearby border crossings – in Jarabulus and Tal Abyad – lead directly into IS-controlled territory in Raqqa, although it is just as likely she paid a smuggler to get her into Syria, experts say.
Whatever her route, Boumeddiene has refocused international attention on Turkey's apparent reluctance to crack down on the passage of insurgents through its territory and its failure to fully support the US-led international coalition against the Islamic State in Iraq and Syria.
Turkey's government, dominated by the country's increasingly strident president Recep Tayyip Erdogan, has reacted with anger over the charges.
It deported 1056 foreigners and placed a travel ban on 7833 in a bid to blunt the recruitment of militants, Interior Minister Efkan Ala recently told local media, although it remains unclear over what time frame these actions occurred.
Turkey argues Western countries should also implement better checks at their own airports and departure points in order to catch radicalised locals as they try to leave for Iraq or Syria.
Regardless, it would have been impossible to catch Hayat Boumeddiene, Turkish officials say.
She was not on any travel watch list provided by France and because she flew to Istanbul via Madrid, French security officials were unaware she had even left the country.
It was only three months ago the United Nations warned that foreign jihadists were flocking to Syria and Iraq on an unprecedented scale, estimating there were about 15,000 insurgents from up to 80 countries including Australia, France, Germany, Britain, Denmark and throughout the Middle East.
So concerned about the flow of fighters through Turkey, the UK Prime Minister David Cameron travelled to Ankara last month to discuss the issue with his Turkish counterpart Ahmet Davutoglu.
His aim, Reuters reported, was to convince the Turkish government and the country's commercial airlines to do more to identify radicalised Britons flying in and out of the region.
Their post-meeting statement laid bare just how much relations between Turkey and its European counterparts, including the UK, had deteriorated.
Cameron revealed the two countries had agreed to little more than to "exchange even more information".
Had France provided "even more information", Turkey now argues, it may have been able to arrest Boumeddiene as she entered Istanbul.
Not long ago on track to join the European Union and seen by its Western allies as a leading example of secularism in an increasingly conservative region, Turkey is on the defensive, its president now pushing the political Islam of the Muslim Brotherhood harder than ever before.
There is no doubt the Turkish government has recently stepped up its intelligence operations and increased its security presence along the Syrian border, says Lina Khatib, the director of the Carnegie Middle East Centre.
But these measures, she says, are new and not comprehensive.
"Although open recruitment [of foreign fighters] has lessened in visibility, the fact is that recruitment is an ongoing issue in Turkey because the government has been cautious about being too forceful in cracking down on the practice."
From the beginning of the conflict in Syria, "Turkey bet on jihadists as a key tool to bring down the Assad regime, which lead it to turn a blind eye to foreign jihadists entering Turkey and going into Syria," Khatib says.
Now Turkey finds itself trapped between its desire to stop the flow of foreign jihadists into Syria and the potential for violent retaliation from extremist groups should it do so, she says.
One fighter from a brigade affiliated with the Islamic Front, a coalition of Islamist rebel brigades not aligned with either al-Qaeda or IS, confirmed to Fairfax Media via Skype that both Jabhat al-Nusra and IS were continuing to actively recruit in Turkey.
The man, who did not want to be identified, was speaking from a house in the south-eastern Turkish city of Sanliurfa where he and and several other fighters were taking a break from the front lines near Syria's Hasakah region.
"Yes, both groups are here, we see them around – they are looking for new recruits and also for people to take," he says.
"Gasoline and ransoms for those who are kidnapped, that's how they make their money, and they are looking for more and more fighters every day."
A lack of trust between Turkey and its European partners had led to a dangerous lack of cohesiveness in intelligence sharing, warned Fadi Hakura, an associate fellow at the London-based Chatham House.
"It is emblematic of the increasing troubled relationship between Turkey and individual EU countries and the EU in general," Hakura says.
"Turkey is a very reluctant participant in the US-led coalition against IS in Iraq and Syria, he says, even though it is the only NATO member that borders both Iraq and Syria.
Before it will fully commit to the coalition, Turkey is insisting the military operations include the removal from power of President Assad, as well as a no-fly zone and a safe zone for refugees along its border.
These demands have been categorically rejected by Washington and by the international coalition's other European partners, Hakura says.
"I don't think that the Obama administration or its European partners will wish to expand the military campaign to include the downfall of President Assad in Damascus, especially in light of those events in Paris – if anything the events in Paris will further fortify President Obama's focus on the Islamic State."
However unlikely a change of heart is, "Washington is the key here … if it changed its mind then European partners would probably do so, but very reluctantly".
Didem Akyel Collinsworth, the International Crisis Group's Turkey analyst, says Turkey's desire to see the overthrow of neighbouring President Bashar al-Assad had driven its policy on Syria.
For the first two years of the Syrian civil war it was "willing to hope that all elements in the opposition could help the Free Syrian Army topple Assad, and therefore was possibly more lenient toward them," she says.
"My observation is … that there has been a change in policy," Collinsworth says.
"Turkey has very clearly included jihadi threat from Syria in the main threats that it faces and on the ground there has been attempts to toughen the border against jihadists."
But, she points out, it is a border that runs to more than 900km that is both porous and difficult to police.
"In addition, we are expecting Turkey to accept all refugees," she says, despite the concern that jihadists may be hiding amongst them.
"I agree with Turkey's complaints against Europe – they are not just failing to share names and flight numbers but also failing to provide information about why these people have been singled out."
Added to that, there is the fear of further spill-over of fighting into Turkey, a danger that is increasing as the IS attempts to advance into Kurdish areas, Collinsworth says.
Turkey's 15 million Kurds were incensed that Turkey did not step in to help the besieged Syrian-Kurdish border town of Kobani in October.
The ensuing clashes between protesters and security forces in southern Turkey left dozens dead and many more injured.
In the meantime, Kurdish military groups – the Kurdistan Workers Party (PKK) that fought a decades-long armed struggle for independence from Turkey, along with its sister organisation, the Syrian-based People's Protection Units (YPG) – have put up a fierce fight against Islamic State attacks in Kobane.
"In October we saw street demonstrations in the south-east, including deadly clashes between PKK/YPG and sympathisers of al-Qaeda and the Islamic State … and also between Turkish Kurds and Turkish authorities," Collinsworth says.
Despite the escalating ethnic and sectarian tensions, there are very few Islamic State supporters in Turkey, she says.
"A poll in November found more than 90 per cent of people in Turkey do not support IS, but at the same time the rhetoric of Sunni victimisation is a very strong sentiment in Turkey and that is shared by some segments of the population as well as the ruling party."
And at least 600 Turks – but possibly more – have joined the fight in neighbouring Syria, leaving no doubt Turkey is in an increasingly fraught security situation, she says.
Officially, Turkey is home to about 1.5 million Syrian refugees. But experts say that number is likely as high as 2 million, most living in private housing, many in the towns and cities close to the Syrian border.
Indeed Prime Minister Davutoglu noted that Turkey has kept its border open to allow the refugees to enter.
By comparison, Australia has reserved 2200 places for those fleeing four years of war in Syria – around 255 have reportedly arrived – while in Britain, 90 people have so far been resettled.
The overwhelming burden of Syrian crises has fallen on its neighbours – Lebanon, where Syrian refugees make up more than one-quarter of its 4 million population, as well as Jordan and Turkey, the three countries alone absorbing more than 4 million people.
"It makes the situation difficult to manage for any country, including European countries who are taking only symbolic numbers of refugees as opposed to the large numbers Turkey is taking," says Assistant Professor Giray Sadik from the Yildirim Beyazit University in Turkey's capital, Ankara.
This is adding to the tensions between Europe and Turkey, he says.
"For more than a year both sides have been playing a blame game … and neither is doing enough," he says.
European countries, in particular, have fallen down on sharing timely intelligence, all the while becoming increasingly concerned about foreign fighters returning from the Syria-Iraq battleground to Europe.
The deadly attacks in Paris – attributed to terrorists linked to both Islamic State and al-Qaeda on the Arabic Peninsula – only confirm this, he says.
It is naive to believe that all the refugees who cross from Syria into Turkey are genuinely fleeing the regime, Sadik says.
"It is almost inevitable that there will be some fighters in the almost 2 million refugees we have hosted since the eruption of the Syrian crisis. There is a need for enhanced security cooperation and also emergency humanitarian cooperation with the EU – on both sides it is far from being satisfactory."
One thing is certain, he says. Turkey, like Europe, is vulnerable to attack because of its borders and the presence of foreign fighters on its soil.
A new security law before Parliament may go some way to improving stability along the border, Sadik says, but ultimately, Europe and Turkey need to mend fences and start talking.
Otherwise every country is vulnerable to Paris-style attacks.
