The ceasefire agreement in Syria between the US and Russia is the most important development in Syria since the Russian military intervention on 30 September last year.

It is so significant because it is an accord reached after 10 months of negotiations between the heaviest and most influential hitters in the conflict.

They should be in a position to persuade or compel their allies and proxies to abide by a truce, however reluctant they may be to do so.

There is a further reason for guarded optimism: this is agreement between the US, a superpower under challenge, and Russia, seeking to regain its superpower status which it lost with the fall of the Soviet Union in 1991.

Neither side can easily afford the failure of their gargantuan diplomatic efforts for peace in Syria. If the peace initiative does collapse, as happened with a previous less-detailed effort in February, then the international authority of both countries will be diminished instead of being enhanced – as it will be if they end the war.

The ceasefire is to begin at sunset on Monday, 12 September. The Russian Foreign Minister Sergei Lavrov says that Syria's President Bashar al-Assad has been informed about the agreement and will comply with it.

Syria's civil war: New ceasefire begins start of Eid

His forces will cease bombing and ground attacks. He does not have a lot of choice because, though his forces are advancing and last week re-imposed a siege on rebel-held East Aleppo, his military superiority over the rebels is not overwhelming and depends on Russian air support.

United Nations aid convoys should now enter East Aleppo where it estimates there are between 250,000 and 275,000 people who are not yet starving, but might be so in future after the Syrian Army and its Shia allies regained control of the Ramouseh road a week ago.

This had been temporarily captured by a rebel counter-attack led by Jabhat Fateh al-Sham, formerly known as al-Nusra and the Syrian branch of al-Qaeda.

The UN has supplies positioned in East Aleppo, but these are being run down and, even before East Aleppo was first isolated on losing control of the Castello Road on 17 July, prices in the opposition enclave had risen by 42 per cent in June alone.

Government-held West Aleppo has a population of about 1,550,000, who have never been entirely cut off, though 485,000 of them depend on UN food rations to fend off hunger.

Under the terms of the agreement reached overnight in Geneva, the Syrian air force and ground troops will cease attacks in all parts of Syria except against Isis (also known as Islamic State).

The Syrian government may not be pleased about this because its recent military successes in Aleppo and Damascus. But the course of the civil war in the months since the first Russian airstrikes on 30 September 2015, shows that, though the government may have got stronger, it does not have a decisive advantage over the opposition.

Overall, the military stalemate continues because each side can bounce back after a defeat by asking for more aid from its foreign sponsors. Thus the rebels got more weapons, financial support and logistical help from Turkey, Saudi Arabia and Qatar this year to counter-balance the Russian intervention.

The obvious priority after the ceasefire goes into effect will be for all sides to cease firing. US-backed “moderate” armed groups are supposed to separate themselves from Jabhat Fateh al-Sham (al-Nusra) whose relabelling is dismissed by the US as a PR ploy.

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After a week, supposing the ceasefire holds, al-Nusra and Isis forces will be targeted by US and Russian airpower whose actions will be coordinated by a Joint Implementation Centre. This body will divide Syria into three “boxes”: those dominated solely by al-Nusra and Isis; those that also contain “moderate” opposition; and those that contain only the latter.

This looks complicated and is even more complex than it looks. One problem is that the armed opposition in Syria is dominated by Islamists and primarily by Isis and al-Nusra, with the latter unlikely to sit still while its former partners make themselves scarce and seek safety under a US-Russian air umbrella. Keep in mind that al-Nusra is not only strong and well-disciplined, but is popular in many Sunni Arab areas, particularly in northern Syria. The movement will be all the more difficult to isolate politically and militarily as it is likely to have prepared itself for this moment.

The US and Russia are today the main players in the Syrian conflict which has been progressively internationalised over the two years since Isis captured Mosul, declared a Caliphate and swept through western Iraq and eastern Syria. The rise of Isis and its terrorist attacks across the world meant that the US and Russia could no longer remain on the side lines, but became more engaged militarily and politically. A significant event that did not happen was effectual engagement in Syria any of the EU states, with Britain and France failing to develop practical policies or risk quarrelling with Saudi Arabia and the Gulf monarchies.

Turkey's two million Syrian refugees Show all 11 1 /11 Turkey's two million Syrian refugees Turkey's two million Syrian refugees There are already over 2.5 million Syrian refugees in Turkey, but their current camps can only hold 200,000 people ADEM ALTAN/AFP/Getty Images Turkey's two million Syrian refugees Turkish citizens protest a new deal, also criticised by human rights activists, which will see refugees who arrived in Greece after March 20 be sent back to Turkey AP Photo/Emre Tazegu Turkey's two million Syrian refugees An estimated 80% of Syrian refugee children already in Turkey are unable to attend school BULENT KILIC/AFP/Getty Images Turkey's two million Syrian refugees Refugee children beg for water near the Turkey-Syria border. Turkey has been accused of illegally deporting asylum-seekers back to Syria BULENT KILIC/AFP/Getty Images Turkey's two million Syrian refugees In Turkey, no-one from outside Europe is legally recognised as a refugee, meaning the 2016 deportations may not meet international legal standards for protecting vulnerable people BULENT KILIC/AFP/Getty Images Turkey's two million Syrian refugees A refugee child cries as she is searched by police at the Syria-Turkey border, where 16 refugees (including three children) have been shot dead in the last four months BULENT KILIC/AFP/Getty Images Turkey's two million Syrian refugees Many refugees are living rough on the streets of cities such as Istanbul or Ankara (pictured) ADEM ALTAN/AFP/Getty Images Turkey's two million Syrian refugees Turkish soldiers use water cannon on Syrian refugees BULENT KILIC/AFP/Getty Images Turkey's two million Syrian refugees Syrian refugees shelter from rain in the streets of Istanbul BULENT KILIC/AFP/Getty Images Turkey's two million Syrian refugees A derelict building housing Syrian refugees in Istanbul Carl Court/Getty Images Turkey's two million Syrian refugees Turkey houses around half of all the refugees who have currently fled Syria Carl Court/Getty Images

The greater involvement of Washington and Moscow has displaced and reduced the influence of regional powers – notably Turkey, Saudi Arabia and Qatar – in Syria. They will presumably be expected to help control those parts of the armed opposition that are not Isis or al-Nusra. Another important question will be the attitude of Iran and the Shia axis that includes Iraq and Hezbollah in Lebanon who want Mr Assad to win a conclusive victory.

Another complication in ending the Syrian war is that different parties and factions are not as neatly divided as might appear from maps published by the media. Local warlords change allegiances as do the larger groupings.

For instance, Isis fighters are reported to have fled Jarabulus on the Syrian-Turkish border after Turkish military intervention, but a source within Isis told The Independent that they had simply shaved off their beards and stayed in the town.

Further south at Manbij, the Syrian Kurdish People’s Protection Units (YPG) have supposedly withdrawn east of the Euphrates on Turkish insistence and under American pressure. But other accounts suggest that the YPG fighters have simply changed out of their uniforms and are still in Manbij.

But, for all these obstacles, this ceasefire has a good chance of succeeding because of the political and military power of the US and Russia.

This will have an immediate benefit for besieged towns and districts with a population of 592,000 of whom about one third are in dire need and where people suffer from malnutrition.