As U.S. warplanes bomb his enemies in Syria’s east, President Bashar al-Assad has set loose his own forces in the west, alarming Washington’s few friends on the ground and potentially undermining the U.S.-led coalition against Islamic State.

U.S. President Barack Obama says Washington’s goal in Syria is to defeat Islamic State without helping Assad’s government. The Arab allies that have joined the U.S.-led strikes are some of Assad’s fiercest opponents.

Nevertheless, after first tamping down the use of its own air power in the initial days of the strikes, Syria’s military has intensified its own bombing against some of the rebel groups in the west of the country that Washington considers its allies. […]