Syria war: Why Turkey's battle for northern Syria matters - BBC News
Anyone who thought that the defeat of IS would simplify the conflict in Syria was wrong.
This video can not be played To play this video you need to enable JavaScript in your browser. Turkish tanks taking part in an offensive in northern Syria

Anyone who thought that the defeat of the Islamic State group would lead to an end or a simplification of the conflict in Syria was wrong.

Just look at Turkey's controversial offensive in Syria's northern region of Afrin, intended to extend Turkey's existing buffer zone inside the country and to evict Kurdish fighters from a broad swathe of territory.

The Ankara government sees the fighters as allies of Kurdish separatists inside Turkey. Indeed, despite various shifts in Turkish policy towards the conflict in Syria, opposition to Kurdish autonomy has been constant and absolute.

The Turks will simply not tolerate what they see as the threat posed by an autonomous Kurdish zone on their southern frontier. And they are clearly willing to use significant force to remove it.

But just how much force, and how far could this conflict in northern Syria go?

The Kurdish fighters have long been trained and backed by the Americans, indeed, they have proved to be the most capable of Washington's allies in the struggle against Islamic State.

And with IS defeated, at least as a territorial entity, the Kurds were able to consolidate control over a considerable region in the north.

It was poor messaging by a US military spokesman speaking about the creation of a Kurdish border force to maintain security in northern Syria that gave Ankara its immediate cause to launch its attack.

While the Americans have subsequently sought to play down the novelty of this border force - characterising it as merely the continuation of existing arrangements - US commanders in the region and Trump administration spokesmen in Washington have not been reading from the same playbook.

A Turkish tank arrives at an army base in the border town of Reyhanli

The military men have been stressing America's continuing support for their Kurdish allies, while officials have been uneasily trying to distance themselves from the Kurds while urging restraint on the Turkish government.

This is an uncomfortable position for Washington: its Nato ally Turkey engaged in fierce combat with its main ally in Syria, the Kurds. And it could get worse.

If the Turkish assault moves eastwards towards the town of Manbij, there is a very real risk of the fighting extending into areas where US trainers and special forces may be based.

For the Americans, the Kurdish fighters remain an important element in their evolving strategy for Syria.

IS may be largely defeated in purely military terms, but Washington's attention is shifting. Its new organising principle in the region is the containment of Iran, which, through its support for the Assad regime, has emerged as one of the few beneficiaries of the struggle in Syria.

The US wants to constrain the Assad government's ability to extend its control over key parts of the country, and it also wants to limit Russia's ability to call the diplomatic shots.

And to do all of these things it needs reliable allies on the ground like the Kurds.

The Kurdish-led Syrian Democratic Forces (SDF) alliance captured the IS stronghold of Raqqa

The crisis in northern Syria shows that the simple focus of US policy on the defeat of IS was insufficient to bring stability to the country.

Indeed, many parts of Syria remain as much dangerous battlegrounds as they ever were. Large areas of the country are nominally under the Assad government's control, but in some cases, they are actually in the hands of semi-autonomous militia forces.

Iran also has its proxies on the ground in significant numbers. Opposition groups affiliated one way or another with al-Qaeda hold significant territory. This is hardly a basis for stability, and could well prove the breeding ground for the next upsurge in Islamist extremism.

It is hard to see how the new focus in Washington on containing Iran in Syria will reduce tensions.

But the Turkish military operation poses huge risks for the Ankara government too. Turkish progress on the ground has been steady but mixed, because of fierce Kurdish resistance and poor weather that has hampered operations.

Dozens of Kurdish fighters and civilians have been killed in the fighting

The fighting is throwing up a series of paradoxes.

Turkey, which a few years ago shot down a Russian aircraft that it said intruded into its territory from Syria, has reportedly done a deal with Moscow to enable it to use its air power in northern Syria. (Russia, which pretty much controls Syrian air space, has not intervened.)

There is also evidence - cited by US think tank the Institute for the Study of War (ISW) - which suggests that Syrian government forces have allowed Kurdish reinforcements to pass through their territory on the way to help fighters in the Afrin pocket.

The recent ISW study also cites an episode earlier this week when pro-Syrian government forces fired upon and halted a large Turkish armoured column that was driving southwards to the south-west of Aleppo through opposition-held territory.

The intent might have been to establish a blocking position hindering future operations by Syrian government forces in the area.

The government in Damascus regards the Turkish operation as a whole as an infringement of its sovereignty. Ankara is eager to ensure that the Assad regime does not give any support to the embattled Kurdish fighters.

New battles are being waged where the interests of the outside players are becoming the dominant factor. Turkey has genuine security concerns about what happens in northern Syria, which the US has tried to acknowledge, and the risks it faces are political as much as military.

Turkish policy towards the Syrian crisis has oscillated back and forth.

Turkish President Recep Tayyip Erdogan says he is prepared to take the fight against Kurdish forces in northern Syria as far east as Iraq

Its long-standing hostility to the Assad regime has softened slightly as it sought both Moscow and Tehran's help to create a diplomatic route to shape the future of Syria, or at least that part of the country closest to its own borders.

That diplomatic effort has largely failed, Russia's recent peace conference in Sochi achieving as little as the more broadly-backed Geneva process has done over successive meetings.

The extent and scale of Turkey's military operations will influence its relations with Russia, Syria and Iran. It will impact its ties with Washington and its wider relationships within Nato.

And it risks accentuating the sense of Turkish independence and drift away from the West which is a growing concern in many of the alliance's capitals.