BEIRUT (AP) -- After five years of carnage, the Syrian proxy war can seem poised to go on indefinitely. But the "cessation of hostilities" engineered by the U.S. and Russia may actually stand a chance, in part because of the weakness of the mainstream rebels fighting President Bashar Assad.


The so-called Free Syria Army - and innumerable other militias roaming the shattered landscape - had clearly struggled to even fight Assad to a standoff. But Russia's intervention on his side in September has given him what now appears to be a decisive boost.


The rebels have been routed and besieged in a string of key locations in recent months. Dozens of their commanders have been killed. Most critically, pro-Assad forces backed by Russian air power are close to encircling the rebels in their main stronghold of Aleppo, Syria's largest city. The ability to supply it with fighters and weapons from Turkey has almost dried up.


As the clock ticks toward a truce meant to take effect at midnight Friday local time, the mainstream rebels find themselves fighting resurgent government forces, battle-hardened Hezbollah troops and a host of allied Shiite militiamen, as well as the Islamic State group.


Kurdish forces have cleared over 10 kilometers (7 miles) of opposition-held terrain north of Aleppo recently and are closing in on the opposition strongholds of Marea and Azaz near the Turkish border.


"It is clear that (the rebels') American and European backers are no longer willing to support them in winning the fight," said Randa Slim of the Washington-based Middle East Institute. "The best they can do is to try to help them get better terms for the deal."


The rebels' main regional backers, Turkey and Saudi Arabia, are distracted with their own internal troubles. After threatening a ground incursion in Syria, the Turkish foreign minister said Monday a land operation has never been on the agenda.


If the fighting goes on, chances are good that many rebels would either surrender or slip away to join jihadi groups like the Islamic State group or the Nusra Front.


Even though the Russians have defended their intervention as aligned with the global campaign against IS militants in northeastern Syria, the opposition says most of the sorties have in fact targeted non-jihadi mainstream rebels and were plainly aimed at dislodging them from areas they held in the northwest and south. Tellingly, the Russians have played no role against the Islamic State group in neighboring Iraq, which is an equal part of the fight for the U.S.-led coalition.


That impressive-seeming coalition, which includes most countries in the Arab world, has indeed scored some victories in Iraq, working closely with the Iraqi army in dislodging IS extremists from important places like Tikrit and Ramadi. A campaign to retake Mosul, the main city held by Islamic State in Iraq, is believed to be imminent.


There have been scant parallel successes in Syria, and the Islamic State is still well-entrenched in Raqqa, the main city the group holds in that country. A key difference: There can be no cooperation by the coalition with Assad's army as long as the civil war goes on. And in any case, that army is occupied elsewhere.


In this way, the Syrian war has - in addition to causing a migrant crisis in Europe and unsettling politics in the U.S. - has also undermined the effort to defeat the Islamic State.


Along with the devastation to Syria itself - where half the people are displaced and over a quarter of a million have been killed - it adds up to an overwhelming global sense that the civil war must end.


If the cessation of hostilities were to become long-term or permanent, or lead to a negotiated transition period, it could be a net positive for some key players.


For Assad, whose demise was widely predicted since the early days of the rebellion, any outcome that leaves him standing in Damascus is a victory. And even though few will admit it, his early claim that his opponents were in large part imported jihadi terrorists will have been at least somewhat vindicated by the rise of the Islamic State.


For the mainstream rebels, avoiding total defeat against a force that included not just the Syrian army but also Iran, Hezbollah and Russia will be presentable as a success that should earn then a seat at a future negotiating table. If the fighting ends now, the coalition of rebel groups will remain in control of much of Aleppo and Idlib provinces, and key areas along the Lebanese border and near Damascus in the south.


Russia will get credit for helping end the war. The U.S. will have avoided a Middle East quagmire and ended a situation that is becoming more embarrassing by the day.


Turkey will be frustrated at Assad's continued presence, but along with Syria's other neighbors of Lebanon and Jordan, it will be relieved at a chance to stem the flow of refugees.


With so many of these refugees now in Europe - sparking popular resentment and even a re-evaluation of the European Union's open internal border policy - there will be a sigh of relief from there as well.


For all the cynicism and perhaps justified pessimism surrounding the U.S.-Russian agreement for a cease-fire, the deal offers the most serious effort yet to halt the 5-year-old conflict. It is based on a U.N. Security Council resolution that endorsed a road map for a process designed to end the civil war and provide a new government in Syria within 18 months. And the U.S. and Russia have put some credibility on the line.


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EDITOR'S NOTE: Zeina Karam is the AP's news director for Lebanon and Syria and has covered Syria since 1997. Dan Perry is AP's Middle East editor leading text coverage in the region. He reported from Jerusalem.


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By AP