Barack Obama's repeated insistence that Bashar al-Assad must leave office – and that there are 'moderate' rebel groups in Syria capable of defeating him – has in recent years provoked quiet dissent, and even overt opposition, among some of the most senior officers on the Pentagon's Joint Staff.

The military's resistance dates back to the summer of 2013, when a highly classified assessment, put together by the Defense Intelligence Agency (DIA) and the Joint Chiefs of Staff, then led by General Martin Dempsey, forecast that the fall of the Assad regime would lead to chaos and, potentially, to Syria's takeover by jihadi extremists, much as was then happening in Libya.