de facto military alliance

Syria and Iraq are now de facto allies against both ISIS and the US

One of the most striking turnarounds in the Middle East is the sudden close rapprochement between Syria and Iraq, who in their moves along the Syria-Iraqi border are increasingly coordinating together against both ISIS and the US.
To be clear, this is a major turnaround.  Since the Second World War Syria and Iraq have more often been in bitter conflict with each other than allied with each other.