Across the Middle East, America's traditional allies are watching with disbelief as Russia and Iran mount a show of force in Syria, and they are wondering how it will end.

The US-led coalition, created to combat the jihadi threat from Islamic State in Syria and Iraq, has been wrong footed by the Russian jets pounding the rebels fighting Syrian President Bashar al-Assad, and by an influx of Iranian forces.

The question on everyone's mind is: will the US and its European and regional Sunni allies intervene to stop President Vladimir Putin from reversing the gains made by mainstream Syrian rebels after more than four years of war?

Few are holding their breath.

Many say, often with vehemence, that the current drama is the consequence of ongoing Western inaction and US retreat at critical moments in an ever more uncontrollable conflict, whose regional dimensions are fast becoming global.

Nobody in the Middle East is counting on US President Barack Obama. The gloomy prediction of most is that a war that has killed at least a quarter of a million people and displaced half the Syrian population is about to get much, much worse.

The conflict has taken a deadly trajectory throughout. It began as a popular uprising against Assad, part of the 'Arab Spring', then became a sectarian war with regional patrons such as Iran and Saudi Arabia backing their local proxies.

Military interventions by Russia and Iran have pushed the war to the brink of a full-blown international conflict.

Faisal Al Yafai, chief commentator at the UAE-based newspaper, The National, recalled the words of David Petraeus, the US general who led the "surge" of American military reinforcements into Iraq in 2007-08 - "Tell me how this ends".

After the Russian "surge" into Syria, he said, "America and its allies now look like the only group without a plan".

He believes the emerging military alliance between Russia and Assad's other main backers - Iran and Lebanon's Hezbollah - does have an idea of "how this ends". The same is true of Islamic State, he said.

The end for the Assad family, he argues, is its survival.

For Islamic State, it is to carve out and consolidate the caliphate it declared in large swathes of Syria and Iraq last year. But for Russia and Iran, it is "nothing less than the replacement of the US-Israel axis with one of their own".

With the Kremlin's creation in Baghdad of a centre to share intelligence among Syria, Iraq, Iran and Russia, a Moscow-backed network now runs from Tehran, through Baghdad and Damascus, and via Hezbollah into Lebanon.

The new axis is taking shape as the US has withdrawn ground troops from Iraq and is winding down its military presence in Afghanistan.

While it continues to police Gulf waterways from a base in Bahrain and maintains an airforce presence in Qatar and Turkey, Washington appears determined to avoid deeper military entanglements in the Middle East.

Analysts and diplomats say the turning point in Syria came two years ago when Obama and his European allies shied away from responding to the Assad army's alleged nerve gas attacks on civilians in the rebel enclaves east of Damascus - even though the US president had repeatedly declared that a "red line".

"It was the point when the Assad regime and mainly the Iranians realized that the Americans are not serious, that they really didn't care enough," Yafai said.

For that reason, he doubts whether Russian intervention will lead to a proxy war in the Middle East with Russia. "You have to ask that question in a different way," he said.

"What would it take to make the Americans intervene? Would it take children and women being slaughtered? Well that happened. Will it take millions of people on the move? That happened. Will it take hundreds of thousands of civilians murdered? Well that happened," Yafai told Reuters.

"America wasn't willing at any point to intervene so why now would it suddenly intervene? It is a free field for Putin and the Russians."

Added to US reluctance, the regional scene couldn't have been more favourable for Iran and Moscow to step in.

Saudi Arabia and its Gulf Sunni allies, the main backers of anti-Assad rebels, are immersed in a war in Yemen against Iranian-supported Houthi rebels, while Turkey is busy with its own Kurdish insurgency.

Turkey and its Gulf allies will most likely respond to the Russian and Iranian build-up by increasing military support for mainstream opposition forces in Syria, rather than risk direct intervention.

But some analysts say Russia may be getting in over its head, entering a treacherous quagmire in Syria before it has got a grip on the conflict it started in Ukraine, at a time when Western sanctions and falling oil prices have hurt its economy.

"This is sheer opportunism," said a veteran former UN official with long experience as an envoy in the region. "They've looked at how (bad) we look and seen an opportunity.

"It's a real gamble, the first time they've sent an expeditionary force away from their 'near abroad' since (the 1979 Soviet invasion of) Afghanistan - and even that was on their borders," said the former envoy, who declined to be quoted by name.
"Putin is trying to regain the loss of Russian clout in the Middle East."

Even Yafai says the idea that Russia can supplant the United States in the region is fanciful.

"They don't have the financial power," he said. "They don't need to be that involved because the Americans are leaving so even a small presence will be enough to have a significant impact."

The Assad government, Syria watchers say, has been lucky with its enemies as well as its allies.

What may alter this calculus, some analysts say, is if Russia and Iran move to recapture areas of north-western Syria seized by insurgents earlier this year.

It is in that area that Russian jets have targeted not IS but other Islamist rebels who are fighting against both Assad and IS with support from Turkey, Saudi Arabia, Qatar and in some cases the United States.

As Reuters reported this week, Iranian troops and their Hezbollah allies plan to use Russian air cover to launch a ground offensive in Idlib and Hama provinces, where the IS presence is minimal.

That risks turning all Sunni factions against Russia, while Putin is already nervous about the presence of large numbers of Chechens on the ground in Syria and about IS ambitions to build up its presence in the northern Caucasus.

Sarkis Naoum, a leading Lebanese commentator on Syria, said that if Russia decided to launch a wide-scale operation in the north, it would lead to a "war on an international scale".

If on the other hand, Iran confined its military role to shoring up and fortifying an Assad-held north-western coastal enclave and the capital, Damascus, and avoided mainstream rebel-held territory close to the Jordanian and Turkish borders, the conflict would probably not escalate much more widely.

"This step (attacking rebels in the north) opens the door to an open-ended war in the region and a declared (Sunni-Shi'ite) sectarian war which could in the long term transform to a second Afghanistan for the Russians, and they won't be able to win it," Naoum said.

Moscow's critics as well as non-IS rebels say the Russian and Iranian intervention will draw more Sunni foreign fighters and jihadis into Syria.

"What will Putin do then?" asked Naoum. "If this battle takes place then Putin would drag himself and the world into a predicament whose beginning is known but whose end is not."-Reuters