THE shells continued to fall across the Turkey-Syria border yesterday.

Ankara's anger over the Syrian mortar strike that killed civilians in the southern Turkish border town of Akcakale manifested itself in a retaliatory artillery barrage and a Turkish Parliament vote authorising troops to cross into Syria should the Government deem it necessary.

It was the latest worrying escalation in what US Secretary of State Hillary Clinton stressed was "a very, very dangerous situation".

Turkey's military response marks the first reported retaliation since the Syrian conflict began in the spring of 2011, despite many Syrian armed actions that have directly impacted on Turkey.

Back in April this year, Syrian regime forces fired across the border during a clash near a Turkish refugee camp.

Then, in June, tensions were again ratcheted up when a Turkish air force plane was shot down near Hatay province.

Throughout this time, despite such provocations, Turkey has held its fire. But that was before Syria's strike on Wednesday, which one Turkish Government spokesman described as "the last straw".

Once again Syria's civil war has displayed its potential to act as the blue touch paper for a much wider regional conflagration.

But just how much of a threat does it pose when it comes to sucking neighbouring countries like Turkey and Lebanon into the vortex of all-out war?

Almost from the very start of hostilities in Syria, Turkey and Lebanon have been embroiled.

In Turkey's case its territory has provided sanctuary for opposition activists of the Free Syrian Army (FSA) and other rebel fighters.

In border towns and cities like Antakya and Gaziantep, Syrian opposition fighters rest and regroup, plan operations and build an effective logistical conduit through which ever increasing quantities of guns, grenade launchers and other supplies are handled. Some 120,000 Syrian refugees are also camped here.

It's no coincidence a number of these same Turkish towns have been subjected to car bombs and other terrorist attacks that many security analysts suspect are intimidatory strikes backed by Syria's Mukhabarat, or Military Intelligence Directorate.

In these same Turkish cities Syrian rebel fighters also remain in constant contact with Western and other intelligence operatives from Sunni Arab countries like Saudi Arabia and Qatar, who act as backers as well as tactical and propaganda advisers.

Last month, CIA director David Petraeus was just one of a series senior intelligence chiefs who visited Turkey to hold talks with officials on the situation in Syria.

Ankara, in turn, has been quick to seize the opportunity the crisis has presented it with in terms of expanding its influence as a regional power.

It's much the same story in Lebanon. Beirut, a city that has long been the refuge for many of the Middle East's dissident groups and activists and a place no stranger to political intrigue and violence, has become a hub for many Syrian opposition factions.

Unlike Turkey, however, some indigenous or Lebanese-based political groups do not take kindly to the Syrian opposition running its war from Lebanese soil.

What is important to remember in Lebanon's case is the machinery of its state is in a precarious situation, because some factions within the Lebanese Government and military support Syrian President Bashar al Assad's regime, and some support the Syrian rebels.

Hezbollah, the powerful Shi'a Islamic militant group that has long had close ties and been supported by the Assad regime, is the most obvious case in point.

Faced with such a scenario, the Lebanese military has been hesitant to get fully involved in the crisis and is focusing more on mitigating the spillover from Syria's war.

That said, Lebanon remains an important battleground inextricably linked to the conflict in Syria.

Syrian rebels depend on supply lines running across the Anti-Lebanon Mountains into Syria, particularly from the border town of Arsal into Syria's Homs governorate and from the traditional smuggling gateway of Deir al Ashaer into the Damascus governorate.

For that reason alone, Assad forces use frequent cross-border shelling and raids to disrupt the supply lines, as they did last month when two Syrian Air Force planes fired missiles into Lebanon, striking the outskirts of Arsal.

For now, though, it is Turkey that finds itself in the headlines and has taken the most far reaching military and political steps to date in reacting to aggression from Damascus.

Having proven its point for the moment, Turkey will likely seek to limit future retaliation measures to striking at Syrian units perceived to have opened fire across the border, unless of course there are continued provocations from Syrian regime forces.

Ankara will also continue to co-ordinate very closely with Nato to ensure it has the organisation's support in case of any significant escalation.

Doubtless too Turkey will also increase the aid it gives the Syrian rebels to help accelerate the pace at which they drive pro-Assad troops away from the northern border.

From the Syrian regime's viewpoint, President al Assad will have to walk the political and strategic tightrope between deterring further Turkish military action and Ankara's aid to the Syrian rebels on the one hand, while ensuring his forces maintain their precarious military grip in northern Syria.

Yesterday, despite Ankara's decision to authorise cross-border action against Syria, Turkey's Deputy Prime Minister, Besir Atalay, insisted the measure was only a deterrent and not a mandate for war.

Dangerous as the current escalation is, the inescapable fact remains that any Turkish military intervention in Syria would be costly both in term of lives and money.

Just how unpopular such a drastic course of action would be was almost universally reflected in the editorials of most Turkish newspapers yesterday.

While the prominent pro-secular daily Milliyet said war should be considered as "one of the last cards" the centrist Aksam, was more clear cut in its message: "Let us scream from the very beginning: No war!".

Such calls for Turkish caution will doubtless provide little comfort to those Syrians battling the al Assad regime.

But then frankly, the alternatives don't bear thinking about.