After two months, the U.S.-led aerial campaign in Iraq has hardly dented the core of the Islamic State group's territory. The extremist fighters have melted into urban areas when needed to elude the threat, and they have even succeeded in taking new territory from an Iraqi army that still buckles in the face of militants.

In neighboring Syria, days of airstrikes have been unable to stop militants on the verge of capturing a strategic town on the Turkish border.

The limited results show the central weakness of the campaign: There is only so much that can be done from the air to defeat an extremist force that has swept over much of Iraq and Syria. The Islamic State fighters have proven elusive and flexible, able to reorganize to minimize the blows. And more importantly, there are almost no allied forces on the ground able to capitalize on the airstrikes and wrest back territory from the militants.