By Paul Taylor
(Reuters) - Turkey's worst nightmares are beginning to come true in Syria - a protracted sectarian civil war on its long southern border with the emergence of a de facto Kurdish-controlled region friendly to its main domestic foe.
The Syrian conflict is also poisoning Ankara's sensitive relations with Iran, Syria's vital regional ally, and Iraq and complicating ties with Russia, undermining a declared policy of "zero problems" with the neighbors.
"Syria has turned Turkey's neighborhood policy on its head," said Sinan Ulgen, a former Turkish diplomat now at the Carnegie Endowment for International Peace in Brussels. "Ankara's approach to the Syria conflict has been a radical departure from traditional Turkish caution."
Yet despite bellicose statements, political support for the Syrian opposition and growing covert aid to opposition fighters, there is little Turkey can do alone to shape the outcome.
"We will not allow a terrorist group to establish camps in northern Syria and threaten Turkey," Prime Minister Tayyip Erdogan told a news conference on July 26, referring to the Kurdistan Workers Party (PKK), which has waged a bloody armed struggle since 1984 in southeastern Turkey.
"If there is a step which needs to be taken against the terrorist group, we will definitely take this step."
It was the latest of a string of warnings that have so far had little traction on the course of a conflict that has wrong-footed Turkish diplomatic ambitions in the region.
Before the crisis, Erdogan cultivated a friendship with President Bashar al-Assad, in stark contrast to Turkey's tense relations with the Syrian leader's father, veteran strongman Hafez al-Assad. The ruling couples even vacationed together.
After a Syrian uprising inspired by the "Arab Spring" pro-democracy movements in Tunisia and Egypt erupted in March 2011, Erdogan tried to use those personal ties to persuade Assad to embrace reform and open a dialogue with the opposition.
He was rebuffed and felt slighted. From November, he began calling for the removal of Assad and Turkey helped the opposition Syrian National Council organize on its soil.
But the Syrian leader is still there, albeit weakened. He is part of a Shi'ite Muslim axis spanning Iran and Iraq and his own minority Alawite sect, uncomfortable for mainly Sunni Turkey.
The faction-ridden SNC, dominated by the Muslim Brotherhood, has yet to provide a credible alternative, and international diplomacy is deadlocked and largely irrelevant for now.
"They haven't really thought this through," Gareth Jenkins, an Istanbul-based researcher on Turkish security policy, said of Turkey's leaders. "It's been 'let's get rid of Assad' without enough thought as to what comes next.
"Now their two nightmare scenarios are starting to materialize: the emergence of some form of Kurdish entity in northern Syria that would clearly be an asset to the PKK and embolden Turkish Kurds in terms of autonomy, and the Lebanon-ization of Syria with a long-running ethnic and confessional civil war with different groups controlling different regions," Jenkins said.
Some 45,000 Syrian refugees have poured into Turkey, straining resources and security in some border areas. With fierce fighting raging in Syria's second city, Aleppo, near the Turkish frontier, a bigger influx looms soon.
Military defectors have set up bases of the Free Syrian Army in southern Turkey, and some are trained and coordinated by Turkish, Qatari and Saudi officers operating from a secret "nerve centre" near the city of Adana, Gulf sources have told Reuters.
Foreign Islamist militants are joining the Syrian fighters crossing the border from Turkey to fight against Assad, with the apparent acquiescence of the Turkish authorities, said Fadi Hakura, a Turkey analyst at London's Chatham House think-tank.
"They (Turkish officials) want to accelerate the downfall of Assad and his regime," Hakura said, when asked about Ankara's attitude to such fighters. "The Turkish government feels it can control the aftermath of a post-Assad Syria."
Turkey officially denies arming the rebels, but several sources say they are receiving Russian-made small arms on Turkish soil, although not the heavier weapons they would need to change the balance of power with Assad's superior forces.
"Looking ahead, the most troubling scenario for Turkey may also be the most likely one: protracted chaos and sectarian conflict, leaving a security vacuum across the border, with an ongoing risk of spillovers affecting Turkish security," Ian Lesser, a former U.S. official and Turkey expert at the German Marshall Fund wrote in mid-July.
That future is already here.
Turkish analysts suspect Assad let the main Syrian Kurdish movement, the Democratic Union Party (PYD), take control of security outside the main cities in the northeast last week to prevent them joining forces with the FSA while enabling him to redeploy state security forces to the main battle zones.
Ankara came close to war with Assad's father in 1998 over the presence of PKK leader Abdullah Ocalan in Damascus and alleged Syrian support for PKK activities in northeastern Syria.
If the PKK were to take root and launch attacks from that area, Erdogan would face strong nationalist pressure to launch military action.
"What could happen is for Turkey to carry out the kind of surgical strikes that it did in northern Iraq in past years if the government has intelligence that northern Syria is being used by Kurdish terrorists," Ulgen said.
But Jenkins said the border area was too flat to provide useful terrain for PKK fighters, who preferred to operate out of mountainous northern Iraq despite Ankara's much improved relations with the regional government of Iraqi Kurdistan.
Turkey talked earlier this year of possibly setting up a safe haven inside Syria for people displaced by the fighting, or establishing a military no-fly zone to protect civilians, but no such operation seems likely any time soon.
The United Nations Security Council is paralyzed over sanctions, with Russia and China blocking any resolution under Chapter VII of the U.N. Charter authorizing the use of force.
Ulgen said Turkey could not take such action alone without either U.N. backing or a strong "coalition of the willing" made up of its main NATO allies, which gave Ankara only lukewarm verbal support when a Turkish warplane was downed in disputed circumstances off the Syrian coast in June.
A NATO source said there was no realistic prospect of the alliance operating in Syria unless Turkey were to come under attack from Syrian forces.
Another constraint on Turkish action is domestic public opinion, which is strongly opposed to military intervention.
Opinion polls conducted by Ulgen's EDAM think-tank show public opinion is ill disposed to any armed involvement in Syria and unconvinced by the government's tough rhetoric, even after the warplane incident.
So despite Erdogan's public warnings, Turkey may remain a prisoner of events beyond its control across the border.
"The truth is that they are stuck," said Henri Barkey, another former U.S. official and Turkey specialist at Lehigh University in Pennsylvania.
"They cannot and will not intervene militarily. All they can do is help on the edges, i.e. allow insurgents free passage, train them, help them organise politically," Barkey told Reuters. "Still this is more than what many others are doing."
(Editing by Ralph Boulton)
(Additional reporting by Ayla Jean Yackley in Ankara and William Maclean in London; Writing by Paul Taylor)