The political centre of gravity in Syria’s conflict is shifting to the east of the country, as the Assad regime expands its control in the west. This is significant for many reasons, not least that eastern Syria is where the vital interests of several regional actors and the Syrian regime are most acute.

Eastern Syria is many things to the different political players in Syria. It is where Syria’s oil is concentrated, a vital factor for a Syrian regime in desperate need of revenues. It is also where the regime can re-establish control over its borders with Iraq, a key aspect of its legitimacy.

To Iran, domination of the east ensures geographical continuity between the Islamic Republic and its allies in Syria and Lebanon, through Iranian-dominated Iraq. As for Russia, retaking the east would be the capstone on its persistent efforts to save Bashar Al Assad’s regime.

If Raqqa is lost by ISIL, the eastern border area is where the group’s militants are likely to redeploy. The cities of Deir Ezzor, Al Bukamal and Mayadin are all controlled by ISIL today, giving the group a geographical presence near Sunni areas of Iraq.

The buildup to the battle for Raqqa is only one factor shaping developments in eastern Syria. Recently, US aircraft bombed a column of pro-regime forces, including Hizbollah combatants, heading towards Tanf, in southern Syria near the Jordanian and Iraqi borders. The column was nearing the so-called de-confliction zone around Tanf, where US special forces are based. However, the area is also considered important for the regime to mount an attack against ISIL in eastern Syria’s cities.

In a further sign of the growing focus on eastern Syria, Russia’s ambassador to Iran said last week that there was a “high probability” that Russia and Iran would agree to base Russian warplanes at an Iranian base in Hamadan province in western Iran, from where to attack targets in Syria. While there does not yet appear to be a final deal between the two, it is difficult not to interpret this as a Russian effort to have a presence around Syria’s east in light of the rapidly developing situation there.

In March, Kurdish officials suggested that the United States was setting up two airstrips in Kurdish-controlled north-eastern Syria, one in Rumeilan and the other south of Kobani. While US officials denied the story, a report in the US military’s Stars and Stripes newspaper appeared to confirm parts of it. The article underlined that the airbase near Kobani was being expanded in preparation for the Raqqa campaign.

This succession of events highlighted that eastern Syria is emerging as an area of strategic rivalry between the many political actors with stakes there. As prospects for a post-ISIL phase in eastern Syria increase, the United States, Russia, Iran and the Syrian regime appear to be positioning themselves to ensure that they can exploit the aftermath.

Tehran’s interest in sending pro-Iranian forces in the direction of Tanf, like the possibility of Russia benefiting from airbases in Iran, suggests that both do not want the foes of Mr Al Assad to retain an uncontested toehold in eastern Syria from where they might pursue their agendas.

The situation in north-eastern Syria remains uncertain, given the presence of the Kurdish Democratic Union Party (PYD), with its armed wing, the People’s Protection Units (YPG). That is why the Syrian regime, Russia, and Iran, may want to move into regions to the south and west of Kurdish-dominated areas. Their fear is that the international coalition fighting ISIL, once the group is defeated, may choose to use its presence in Syria as leverage to push for Mr Al Assad’s removal.

The Syrian regime has been very sensitive to developments in the east of the country since 2011. It first ceded its positions to the PYD in Hasakeh governorate in order to prevent anti-Assad Arab Syrian groups from taking over of the territory. In Deir Ezzor, the regime has fought hard to preserve its presence in parts of the city and in a nearby airbase, precisely because it could not afford to be eliminated completely from oil-rich areas along its strategically-vital boundaries with Iraq.

These imperatives are as alive as ever as the situation in the east appears to be moving towards a fundamental shift, assuming that ISIL is defeated farther north. With the regime consolidating its military gains in western Syria, it is in a better position to consider reimposing its writ elsewhere. That is Mr Al Assad’s main objective, to claw back the entirety of the country that his father left him.

Nothing indicates that Iran and Russia are thinking any differently than the Syrian president. He may be greatly dependent on them for his military gains, but that only makes them more eager to help him.

Mr Al Assad’s eyes are firmly on eastern Syria, but the complexities of the region mean that its recapture may not be so easy. Without the east, however, the Syrian regime will not be able to claim victory in its war.

Michael Young is a writer and editor in Beirut

On Twitter: @BeirutCalling