Today’s reality is that Iraq and Syria as we have known them are gone. The Islamic State has carved out a new entity from the post-Ottoman Empire settlement, mobilizing Sunni opposition to the regime of President Bashar al-Assad and the Iran-dominated government of Iraq. Also emerging, after years of effort, is a de facto independent Kurdistan.

If, in this context, defeating the Islamic State means restoring to power Mr. Assad in Syria and Iran’s puppets in Iraq, that outcome is neither feasible nor desirable. Rather than striving to recreate the post-World War I map, Washington should recognize the new geopolitics. The best alternative to the Islamic State in northeastern Syria and western Iraq is a new, independent Sunni state.

This “Sunni-stan” has economic potential as an oil producer (subject to negotiation with the Kurds, to be sure), and could be a bulwark against both Mr. Assad and Iran-allied Baghdad. The rulers of the Arab Gulf states, who should by now have learned the risk to their own security of funding Islamist extremism, could provide significant financing. And Turkey — still a NATO ally, don’t forget — would enjoy greater stability on its southern border, making the existence of a new state at least tolerable.

The functional independence of Kurdistan reinforces this approach. The Kurds have finally become too big a force in the region for Baghdad or Damascus to push them around. They will not be cajoled or coerced into relinquishing territory they now control to Mr. Assad in Syria or to Iraq’s Shiite militias.