The Syrian Civil War entered its sixth year in March, and the landscape has certainly shifted. The Islamic State, still used as a tool for fear mongering by all sides involved in the conflict, is on its last legs. In Mosul, IS forces have been completely surrounded by the Iraqi army and its allies. Raqqa, the other major urban hub controlled by IS, is now also under siege by Syrian Kurdish forces. Without control of either of these cities, IS will be cut off from supplies, recruits, finances, and will have lost its most defensible positions. Though IS will likely remain an insurgent and terrorist network in the region and around the world for a decade or more, the group's hold on territory (which also serves as part of its claim to being the Caliphate) will be virtually eradicated within the next two years.
Unfortunately this is not the end of Syria's problems -- the divisions and factions which started this war remain present and actively engaged in conflict, serving as proxies not only for a struggle between Russia and the US, but also between Iran and Saudi Arabia. The central question for strategists in the region, therefore, has changed: it is no longer "How do we defeat the Islamic State?" but instead, "Who gets the spoils?".
This question may help to explain why Russia this week announced that it would consider all US-coalition aircraft west of the Euphrates to be targets. As the linked CNN article reports, this is not the first time Russia has made that claim, having said the same thing months prior after the US cruise missile strike on a Syrian airfield believed to house chemical weapons. It is just as unlikely now as it was after the first time Russia made this claim that it would actually risk open war with the United States or it allies over this incident. What is significant, however, is the geographic boundaries it attempts to draw: claiming all territory west of the Euphrates to be under exclusive Russian/Syrian government control, in spite of the fact that all of the non-Kurdish Syrian Opposition forces lie west of the river, and the fact that the Euphrates runs straight through the heart of what remains of IS territory.
Both sides know that they will not gain full control of Syria through military means any time soon without risking dangerous escalation between the US and Russia. As such, strengthening their bargaining positions for potential post-IS peace & settlement talks has taken on a new priority for all the factions involved. Though the conquest of Aleppo was a big win in favor of Assad, the US-backed coalition now stands to take Raqqa and most of the remaining IS territory in Syria, on both sides of the Euphrates. This should frighten Assad, regardless of whether this will simply be used as leverage in a bargaining scenario, or be used as a strategic advantage should the war continue, or should the US-led coalition decide that Assad must go after all. Russia's bluff that it has such firm military control over the Euphrates is an attempt at preempting the coalition's advance through IS territory, and a way of signalling which division of Syria it sees as preferable should a ceasefire go into place.
Regardless of whether the US believes or calls the bluff, it seems likely that Syria will be divided for a long time after the defeat of IS. Neither side is willing to risk the potential nuclear escalation involved in direct military confrontation. And any peace negotiations between Assad & the opposition seem doomed to stagnate for years before any real progress happens, thanks to both Assad's crimes against the people of Syria and the rebels' internal divisions. Each side is now focused on carving out its own sphere of influence before that stalemate falls into place.
Russia and the United States, unwilling to confront each other directly, but each occupying partitions of a divided state, after the defeat of an enemy infamous for its atrocities? Sitting in Berlin, this story reeks of familiarity. Though the narrative of a 'new cold war' has been dominating headlines since at least the annexation of Crimea, with all the other proxy conflicts in the Middle East and beyond, one is left wondering how cold this new cold war will really be.