President Barack Obama has authorized using air power to defend a new U.S.-backed fighting force in Syria if it is attacked by Syrian government forces or other groups, raising the risk of the American military coming into direct conflict with the regime of President Bashar al-Assad.

U.S. officials said the decision ended a monthslong debate over the role the American military should play in supporting its few allies on the battlefield in Syria. Administration officials had been deeply concerned that defending the Pentagon-backed force could inadvertently open the first open conflict with the Assad government, which has denounced the U.S. program.

Though the new rules allow Pentagon strikes to defend the U.S.-allied force against any regime attacks, U.S. military officials played down the chances of a direct confrontation, at least in the near term. The newly trained force has committed to fighting Islamic State, not the regime, and won’t be fielded in areas the regime controls. U.S. officials say they believe the regime won’t challenge the new force. […]