The American air campaign against ISIS seems to have emboldened the radical Islamic fighters, making them both more prudent and deadly on the ground.
The fighters of ISIS know how to adapt to various circumstances, and they have proceeded by sheltering their weapons more and hiding among civilian populations. On the ground, the Iraqi army seems to be unable to retake any territory even with the U.S. air force behind them.
Nor should we think ISIS will be automatically defeated. It was not that long ago historically when the United States was stymied in Vietnam despite throwing the force of our entire army into the battle. And if ISIS continues along the current course, they will take Baghdad in a few months. Most likely, they will do it in a sudden kamikaze push and surprise us again.