BYE-BYE, BABY:

One evening at the end of March, a Syrian rebel leader returned from a meeting across the border in Turkey and called an urgent gathering of his commanders. The five men turned up at their boss's house in Idlib province expecting to receive the same pleas for patience that they had always heard and more grim news about cash and weapons being hard to find. This time, though, they were in for a shock.

"He arrived looking eager," said one of the commanders. "That caught our attention straight away. But when he started to speak, we were all stunned."

The leader, who asked that his unit not be identified, said he told his men that the grinding war of attrition they had fought against the Syrian government since early 2012 was about to turn in their favour.

"And the reason for that was that I could now get nearly all the weapons I wanted," he told the Observer. "For the first time they were not holding anything from us - except anti-aircraft missiles. The Turks and their friends wanted this over with."

The leader says he explained that they and every other opposition group in the north, with the exception of Islamic State (Isis), were about to be beneficiaries of a detente between regional powers who had agreed to put their own rivalries aside and focus on a common enemy - the Syrian regime.

The agreement had been secured by Saudi Arabia, which had resolved to do all it could to end the Bashar al-Assad regime and, more important, to quash the ambitions of Assad's main backer, Iran, to control the course of the war. It signified a new phase in an age-old tussle between regional rivals for power and influence that was to have profound ramifications for the way the war in Syria, and proxy standoffs elsewhere in the Middle East, were to be fought.

In early March, senior regional figures had been summoned to Riyadh by the newly crowned King Salman to hear his plans for the region. The Turkish president, Recep Tayyip Erdogan, was one of the first to arrive. Qatari officials and Gulf Co-operation Council leaders soon followed.

His message was threefold: first, there was to be no more division along regional lines, which had seen the Muslim Brotherhood-aligned governments of Turkey and Qatar pour support into allied Syrian groups, while Saudi focused on more mainstream outfits. Second, Riyadh would agree to send gamechanging weaponry to northern Syria in return for guarantees of coordination and discipline. And, finally, the US would not stand in the way. "Quite frankly," a Saudi official told the Observer, "it would not have bothered us if they had tried to."

Within weeks, the new push had paid clear dividends. Armed with dozens of guided TOW anti-tank missiles, which could take out regime armour from several miles away, opposition groups - among them al-Qaida's affiliate in Syria, Jabhat al-Nusra, which is proscribed by the US as a terror group and has long been viewed warily by Riyadh - started advancing into towns and cities that they had not dared to attack until then.

The results were shocking. The regional capital of Idlib fell within days. Several weeks later, the nearby town of Jisr al-Shughour also fell to an amalgam of jihadists and moderates who had kept their end of the bargain.

Now, the agricultural plains that stretch towards Syria's third and fourth cities, Homs and Hama, appear more vulnerable than at any time since mid-2012. So does the Mediterranean coast, and the mountains to the north, which are the heartland of the Alawite sect, dominant in Syria's political and security establishment. And to the east, Aleppo, which had been under serious threat of being encircled by Assad's forces six months ago, now looks much more likely to fall to the rebels.

In Ankara and Riyadh and even Baghdad and Beirut - both nominally Assad allies - there is now a strong sense that the war is going poorly for the regime.

Follow @brothersjudd