After marathon takes, and with caveats and reservations, the U.S. and Russia announced a plan for a truce and military coordination in Syria — and it's easy to find reasons it could fail.

As Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov acknowledged, the agreement sits on a foundation of profound mistrust and comes after a similar effort, introduced in February, gradually fell apart.

But diplomats have pushed hard because they see it as the only realistic path toward larger goals in Syria: reducing overall violence, winning permission from the regime to aid besieged people, resuming stalled peace talks and effectively beating back extremists groups.

Acknowledging the difficulties, one U.S. administration official this week said, "We look around at the range of possible solutions and ... this remains, we think, the most promising way that we see in front of us to try to get this horrible situation to a better place."

On the ground in Syria, some rebel fighters, who long ago lost faith in the earlier deal, say they would probably participate in another ceasefire after bruising months of airstrikes and sieges.

Seeking reasons why this deal would succeed, insiders point out this plan is broader than the ill-fated deal in February.

Kerry and Lavrov say that Russia has persuaded its ally, the Syrian government, that its air force will not fly combat missions in areas where the opposition is present — areas which will be delineated by the U.S. and Russia.

U.S. and Russia plan to work together

Lavrov says if a ceasefire holds for a week, and both the regime and the opposition allow access to aid — particularly to the northern city of Aleppo — then the American and Russian air forces will work together through a joint operations room to target ISIS and other extremists.

A letter from the U.S. special envoy to Syrian rebels before the deal was finalized said the Russians and U.S. won't attack more moderate rebel forces, even when they were working closely with an al–Qaida affiliate.

For Russia, analysts say the main incentive to make this work would be the prospect of direct military-to-military cooperation with the U.S. — a powerful draw that would further legitimize Russia as a key player in the region and ease its isolation.

Kerry struck a realistic note even as he announced the new initiative.

"It is an opportunity and not more than that until it becomes a reality," he said.

His many notes of caution are understandable. The U.S.-Russia deal in February also began with a cessation of hostilities — a quasi-truce which restricted fighting to frontlines and said offensives should only be directed at the Islamic State and Syria's al-Qaida affiliate.

A brief lull

There was a lull in fighting initially, and it was meant to be accompanied by the Syrian government's lifting of tight restrictions on aid to the many rebel-held areas it surrounds, and then peace talks.

But all three pillars of the agreement crumbled.

Negotiations on delivering aid to rebel areas are ongoing but glacial. The regime maintains control on quality and quantity of aid. In Daraya, near Damascus, one abortive delivery was accompanied with a hail of artillery.

Peace talks that began in Geneva in February were halted in April. The opposition walked out, citing a steady increase in regime attacks on civilian areas. Violence has stayed low in the south but is now raging in the north and west, with civilians in rebel and regime areas reporting heavy casualties.

On peace talks, the gaps are vast. The main Syrian opposition group reiterated in London this week that they will insist in negotiations that President Bashar Assad must step down within six months of a transitional process beginning. Assad and his allies are equally insistent that he must remain in power.

Prospects for aid access are also dim, including for the eastern part of Aleppo, where the opposition has been in control in recent years. The area has recently been intermittently besieged since Assad's forces cut off a supply line for hundreds of thousands of people. The U.N.'s negotiators gave a dismal press conference Friday.

"Not just August, but actually July was also extremely disappointing," said the emergency relief coordinator Stephen O'Brien. "The convoys are not going, are not rolling, at the moment in Syria."

Syrian government has employed sieges

The government's siege tactics and denial of aid have been working for the Assad regime. Rebels negotiated a surrender and left one of their strongholds, Daraya, near Damascus, and are in the process of doing so in two more areas.

And the cessation of hostilities, the pause in fighting that is the basic premise of this deal, is also not guaranteed. It has to hold before any of the more complicated steps can be taken. But both the regime and rebels are currently fighting hard, particularly around Aleppo in the north, but also in other places, including close to the central city of Hama.

Stumbling blocks in the negotiations were believed to hinge in part on disagreement between the Russia and the U.S. on which groups are dangerous extremists, and therefore are still considered legitimate targets despite the truce in other areas.

The group considered to be Syria's al-Qaida affiliate — despite a rebranding — is probably now more integrated with other forces than it was a few months ago.

And some analysts question the wisdom of negotiating with the Russians at all. Obama spoke this week about "gaps of trust." Syrians in opposition areas believe Russian airstrikes deliberately target civilian infrastructure, in an effort to depopulate their areas.

When Mark Ratney, the U.S. special envoy for Syria, wrote to opposition forces in an effort to convince them of the deal, he said, "Allow me to be frank with you: Dealing with Russia was very difficult, because it was very difficult to hold these talks with the Russians when they were killing Syrians daily."

In general, rebels told NPR they would abide by a new truce, but only if promises to allow access, especially to Aleppo, were kept. One rebel spokesman, Bassam Hajj Moustapha of the Nourreddine al-Zinky movement, balked at the prospect of a Russian-led truce while eastern Aleppo is still besieged. He called it a "humiliating and degrading agreement."

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