LONDON — LAST week, President Obama virtually declared war on the Islamic State in Iraq and Syria. But it is hard to reconcile the seemingly urgent need to confront the threat posed by this organization with the chosen means of doing so.

By opting to support the “moderate” Syrian opposition and running the risk of an open confrontation with President Bashar al-Assad’s regime, the West appears to be primarily appeasing Arab Persian Gulf allies that have turned the overthrow of Mr. Assad into a policy fetish that runs against any rational calculation of how to defeat Islamist terrorism.

The persistent belief in Western policy circles that there is a “moderate opposition” in Syria — reiterated at the close of a NATO summit meeting in Wales on Sept. 5 — warrants serious scrutiny. The very notion of a “vetted” opposition has an absurd ring to it. It assumes that moderation is an identifiable, fixed element that can be sorted out from other, tainted characteristics. It further presumes that the vetting process will not stain those being vetted. It takes as a given that Western-backed opposition will prevail and in turn provide the basis for a happier and better Syria.

There is little to support any of these beliefs. The most effective forces on the ground today — and for the foreseeable future — are decidedly nonmoderate. This is not primarily because the West has let down the Syrian opposition, but because the conflict now sweeping through the Levant is grounded in elements that have little to do with the presumed struggle between moderation and extremism.