Reports that the Syrian Democratic Forces (SDF) have warned Iraqi Shia militias not to enter SDF-controlled territory are, on the face of it, a potential addition to the complexity of the conflict in Syria. The SDF is dominated overwhelmingly by the People's Protection Units (YPG), a Syrian Kurdish militia which is politically and ideologically aligned to the Kurdistan Workers' Party (PKK). The YPG is the key US ally in Syria and the spearhead of the imminent assault on the Daesh stronghold of Raqqa.
While it remains to be seen to what extent this belligerent statement by the SDF reflects official YPG policy, there can be little doubt that the inspiration for the move was the United States and the intended recipient of the message is Iran. It is the latest sign of an aggressive US posture in Syria vis-à-vis Tehran and forces aligned with the Iranians. In this instance the primary US objective is to block Iran from constructing a land corridor from Iraq all the way to the Syrian Mediterranean coast.
However, this latest escalation will reinforce Tehran's determination to press ahead with its plans and to reap the rewards of its considerable investment of blood and money in the Syrian conflict. Iran holds the advantage in this tug of war, for not only is Tehran willing to risk military confrontation but it also has more assets on the ground, as well as greater long-term strategic resolve.
The spectre of Iraqi Shia militias formally organised as Popular Mobilisation Units (PMU) on the Iraq-Syria border is a clear sign of the shifting balance of power in Iraq in the wake of Daesh's military collapse. Whilst the biggest winners of this collapse in north-western Iraq are the Iraqi Kurds, in the context of the US-Iran rivalry in the country, the Iranians have managed once again to outmanoeuvre their American foes. Both powers have invested considerable resources in the fight to dislodge Daesh from Mosul with a view to expanding their own influence in Iraq.
As the Iraqi army and specialised units of the federal Iraqi police force have borne the brunt of the fighting inside Mosul, by contrast the PMUs made a strategic push for areas west of the city. In view of the PMU's multi-level connectivity to the infrastructure of Iranian influence in Iraq, this decision was almost certainly made in Tehran.
Control of sensitive points on the Iraq-Syria border is a key objective of the PMU, for reasons of deterrence and power projection. From a deterrent point of view, the control of borders creates a wider sense of security and deters Daesh and its allies from regrouping in nearby areas. In terms of power projection, the PMU needs to create an impression that it has the ability (if not the volition) to cross the border brazenly in organised form with a view to supporting the Syrian government and allied forces.
The volition aspect is important as hitherto the PMU has not formally crossed the border, even though large numbers of Iraqi Shia militiamen have undoubtedly crossed in order to augment their Syrian allies in multiple battle zones.
The latest belligerent statement by the SDF notwithstanding, in reality there is little risk of an imminent conflict between the PMU and the YPG. Despite their deep political and ideological differences, both sides share pragmatic common interests, not least antipathy towards Syrian rebels and jihadists.
Moreover, inside Iraq the PMU is allied to the Sinjar Resistance Units (SRU), an ideological compatriot of the YPG in so far as both groups are extensions of the PKK. There are reports that the PMU mopping-up operations at key points on the Iraq-Syria border are being supported actively by the SRU.
In grand strategic terms, PMU presence at key points of the Iraq-Syria border essentially means that Iran has established partial control over this sensitive border. Whilst this marks an expansion of Iranian influence in Iraq, its real significance lies in the messages it sends across the border in Syria.
In immediate terms, it is a response to recent US escalation, notably last month's bombing of a pro-Syrian convoy close to al-Tanf. It is a sign of resolve by the Islamic Republic and a message that it will not sit idly by as the US sets about dominating north-east Syria by proxy, principally via the SDF.
At a deeper strategic level, it signifies Tehran's resolve to construct the land corridor linking Iran to Syria. This project is vital to Iranian national security as well as wider economic and commercial considerations. Furthermore, the Iranians view this corridor as payoff for their unwavering support to the Syrian government from the onset of the conflict.
In view of the fact that a central aim of US concentration of forces, advisors and proxies in areas close to the Iraqi border appears to be to derail the Iranian land corridor project, a clash at some point is inevitable. De-escalation on this front by either party sends an unmistakeable message of weakness with wider repercussions for control of post-conflict Syria.
Read: Iran to change course of land corridor from Tehran to the Mediterranean
Within Syria, though, the US is playing catch-up following years of relative disengagement. Absent the deployment of a sizeable military force to eastern Syria, the US is set to lose this tug of war with Iran. For a start, local US allies are neither reliable nor necessarily durable. As an umbrella organisation, the SDF is bound to fragment at some point, most likely following the military defeat of Daesh in Syria.
Whilst the core of the SDF, namely the PKK-aligned YPG, will remain a cohesive force, the durability of "Rojava" as a political entity inside Syria is not a valid proposition. All the key stakeholders in the conflict, notably the Syrian government, the Syrian opposition, the Syrian rebels on the ground, and not least Turkey, are fundamentally opposed to a PKK statelet inside Syria masquerading as Kurdish autonomy.
The battle for eastern Syria will begin in earnest following Daesh's ejection from Raqqa and Deir ez-Zor. Absent a dramatic change in the confluence of perspectives, interests and events, US- and Iranian-aligned forces are set for a major clash in the region.
The views expressed in this article belong to the author and do not necessarily reflect the editorial policy of Middle East Monitor.