Perhaps because it has been anticipated for so long, the ghastly revelation that Islamic State militants are now deploying chemical weapons against their Kurdish foes did not shock Western observers. When it was revealed that ISIS had deployed not merely chlorine gas but the disfiguring blister agent mustard gas, it triggered only modest revulsion in the Western press. Our muted reaction is its own tragedy. We are inured to ISIS’s barbarism. We have come to terms with its existence. We no longer flinch when men are drowned or burned alive for the cameras. We see their child executioners as a reflection of our impotence, and we turn away. We have come to accept the use of weapons of mass destruction in civilian quarters and on battlefields as a lamentable new normal in the Middle East. This, perhaps more than any of President Barack Obama’s dreadful foreign policy legacies, is among his worst. We have become habituated to horrible conflict. What’s more, we have fooled ourselves into believing that we are powerless to do anything about it. That is a comforting lie, and it must be dispelled if the geopolitical order for which our grandfathers fought is to be salvaged for our children.