ACROSS years of war and at an extraordinary cost, the United States built an army that was supposed to prevent jihadists from gaining a sanctuary in the heart of the Middle East. It had American-trained leaders, American-made weaponry and 250,000 men under arms — far more troops and firepower than any insurgent force that might emerge to challenge it.

That army was the Iraqi Army, and we know what happened next: The Syrian civil war spilled over into Iraq, jihadists first found a foothold and then led an insurgency against the Iraqi military, and the jihadists won. American-organized units were routed; American-trained soldiers fled; American-made weapons fell into the hands of the Islamic State, the self-declared caliphate with which we ourselves are now at war.

Perhaps, just perhaps, there might be a lesson here about how hard it is to conjure up reliable allies amid the chaos of the current Middle East. But if so, we seem determined not to learn it, since our official strategy for fighting the Islamic State involves basically trying the same thing again, this time on the cheap: inventing allies, funneling them money and weaponry, and telling ourselves that it will all work out.

Those allies are the “moderate” and “vetted” — euphemisms for “not as scary as the other guys” — rebels in Syria, whom Congress voted last week to finance and train and arm. As fighting forces go, they promise to be rather less impressive than the last army we trained, since if all goes well just 5,000 rebels will be ready for the fight this year, or about one-sixth as many fighters as ISIS now has under arms. (And those odds get even longer when you consider that the rebels intend to use our weapons to fight the Assad regime as well.)