BEIRUT, Lebanon — American-backed forces have begun an assault on Raqqa, the Islamic State’s hub in northern Syria, and signs are that they could capture the long-sought target with relative ease. Yet the militant group’s commanders, who have already withdrawn their toughest forces from the city, and most everyone else in Syria’s multifaceted war are looking ahead to an even more decisive battle in the south.

There, a complex confrontation is unfolding, with far more geopolitical import and risk. The Islamic State is expected to make its last stand not in Raqqa but in an area that encompasses the borders with Iraq and Jordan and much of Syria’s modest oil reserves, making it important in stabilizing Syria and influencing its neighboring countries.

Whoever lays claim to the sparsely populated area in this 21st-century version of the Great Game not only will take credit for seizing what is likely to be the Islamic State’s last patch of a territorial caliphate in Syria, but also will play an important role in determining Syria’s future and the postwar dynamics of the region.

With the stakes so high, the United States, Iran and Russia are all scrambling for advantage. They are building up their forces and proxy fighters and, increasingly, engaging in inflammatory clashes that threaten to escalate into a larger conflict.