Fighters allied with the United States and Russia, long on opposing sides in the Syrian civil war, are both zeroing in on Islamic State’s center of gravity.

After routing the self-declared caliphate in the ancient city of Palmyra March 27 with the help of Russian air power, the Syrian army’s next major objective is cutting off the terror group’s main supply route between Iraq and Syria. Kurdish-led forces backed by the United States are also getting closer to Islamic State’s capital of Raqqa, raising the possibility of a pincer movement that would bring the U.S. and Russia into a de facto alliance. That would have the effect of bolstering Syrian President Bashar Assad’s hold on power, analysts say.

As the United States and Russia step up efforts to complete a peace deal to follow a partial cease-fire they brokered in February, their interests are converging in fighting the radical Islamist group even amid American reluctance to legitimize Assad. A successful campaign could prove critical for Europe as it grapples for a solution to rising terrorist attacks and the biggest refugee crisis in Europe since World War II. […]