To stop it now would require a giant new military effort, which is vanishingly unlikely to happen unless—until—this disaster comes home to our doorstep in some spectacular way. But to stop it along the way would not have required much, if any, direct military force. It would have meant keeping a few tens of thousand of trainers, advisors, and special forces in Iraq to support the government and back up our influence, plus a whole lot of vigorous diplomacy. But the hallmark of this administration is that they came in talking a lot about how they preferred diplomacy to military action—and then showed virtually no interest in accomplishing anything with diplomacy.
Robert Tracinski is an outstanding thinker and observer. This is worth the full read.