The surrender of rebel forces in south-west Syria, following the fall of eastern Ghouta and other regime successes, strengthens the grim expectation that nothing can now deny Bashar al-Assad a final victory – of sorts – in Syria’s civil war.

But the physical restoration of territory does not necessarily translate into restoration of political authority. For Assad, besieged by predatory friends and neighbours and shunned internationally, the larger question is: having survived the war, can he survive the peace?

Assad is deeply in hock to Russia, whose 2015 military intervention saved his bacon, and Iran, which also came to his rescue. He remains dependent on their support, for now. Both have paid a high price for their stake in shaping Syria’s future. They will not quickly relinquish it.

There is also the Turkish problem. In a 2016 cross-border operation against Syrian Kurdish forces, Turkish troops carved out an enclave west of the Euphrates that they continue to occupy. Turkey has since seized control of additional territory in Afrin, north of Aleppo.

Autonomous Kurdish militias still hold sway in east and north-east Syria, supported by about 2,000 US troops and air power. The US is focused on hunting Islamic State terrorists, but its presence gives it skin in the bigger strategic game.

And then there is Idlib province, in north-west Syria, to which the southern rebels are fleeing. Idlib has become the final refuge for more than two million displaced civilians. It is also a base for hardline jihadists and secular regime opponents.

If Assad is to end the war, Idlib is where he and allies must strike next. It is a prospect that horrifies the UN and aid organisations. They fear a repeat of murderous Ghouta-style airstrikes, barrel bombs and chemical weapons attacks.

Nobody knows for certain what was agreed on Syria when Donald Trump met Russia’s president, Vladimir Putin, in Helsinki. That includes people who really should know, such as General Joseph Votel, commander of US forces in the Middle East. He admitted he had received “no new guidance”.

But certain points of US-Russia agreement are becoming clearer. The most important is that both believe additional instability, especially spanning the Syria-Israel border, must be avoided. That means, in practice, US acceptance that Assad will stay in office – and Trump seems to have meekly complied.

In return, the US – egged on by Benjamin Netanyahu’s government in Israel – wants the Russians to ensure Iranian forces in Syria are kept away from the Israeli border. Netanyahu would prefer them expelled altogether, but that will have to wait until Trump’s accelerating war of attrition against Iran gains full force this autumn.

By withdrawing support for the southern rebels and abandoning the “de-escalation zone” created last year, Trump helped Putin and Assad hasten last week’s capitulation. He also tamely tagged along in Helsinki when Putin took the lead in stressing the need to restore the 1974 Golan Heights ceasefire.

Trump had probably never heard of the 1974 pact. Even so, he again appeared content to cede the geopolitical initiative to Putin. That will be news to many in Congress, and the Nato allies, which have backed Assad’s overthrow since 2011.

New, too, was Trump’s claim that the US and Russian militaries were co-ordinating closely in Syria. His statement ignored incidents when Russian mercenaries have fought US special forces and warplanes have almost collided.

Seizing a chance to further legitimise his Syrian ascendancy and normalise bilateral relations, Putin declared Syria could become a “showcase” for bilateral military collaboration. General Votel later pointed out this was against US law.

Trump’s vote of confidence in Assad and his Russian backers will please the Saudis and other Sunni Muslim Gulf governments, whose main beef (like Israel) is with Tehran. Iran’s mullahs will wonder, reasonably enough, whether Assad is preparing a stab in the back.

Once the war is over, Assad is unlikely to want Syria to continue to be used as a forward base for Iran’s regional ambitions, any more than he would welcome Turkey’s ongoing northern occupation. Nor can Iran wholly trust Putin. But its ability to push back is diminishing, undercut by a growing economic crisis and deepening international isolation. Iran may find foreign entanglements such as Syria and Yemen increasingly unaffordable, politically and financially.

For its part, Israel has an interest in a stable Assad regime, if that helps to reduce the Iranian threat. As the war heads towards its Idlib denouement, Putin is on course to emerge a clear winner, with Assad trailing in his wake. The big losers look like being Iran, American global leadership and, most of all, the Syrian people.