Thirty minutes of terror on the streets of Paris looks to become the catalyst for a broad shift in international politics with implications that could last for years.

Much of the focus in the West over the past year has been on a perceived growing threat from Russia. Terrorism was a real, but containable problem. Moscow’s new aggressive military posture in Ukraine and beyond, on the other hand, posed a more serious threat.

But with a series of well-coordinated strikes, Islamic State put the threat of terrorism back at the center of the international agenda.

And Russia, far from a nuclear-armed enemy, instantly presented itself as a partner—one with a plan for immediately tamping down the threat.

Moscow’s strategy—to back the regime of Syrian President Bashar al-Assad, at least temporarily, as the best way of tackling Islamic State—has been unpalatable to the U.S. and, up to now, France, which has been one of Mr. Assad’s fiercest international critics. That position could well be changing.