When Iraq was fighting jihadis and Assad was fighting jihadis and non-jihadi rebels, proponents of intervention could make a case to support Iraq, naturally. We fought there with the Iraqis and the jihadis are our enemies, too. We didn't do that. But the argument lay dormant until President Obama revived it in justifying our limited intervention after the recent dramatic jihadi gains.
Supporting the rebellion in Syria against Assad could get around the jihadi problem by seeking to arm the non-jihadis to help overthrow Assad and strengthen the non-jihadis for the follow-up war to defeat the jihadis rather than welcome them into the new government.
I'm not saying that this was an easy plan. But it at least defeats a major enemy with lots of American blood on his hands--Assad and his Alawite backers--and is more defensible as a sequence than arming the Soviets to defeat the Nazis in World War II and then waging a long Cold War to defeat the Soviets. Nobody has said we need to arm the jihadis, after all--just risk that jihadis could get arms we send to non-jihadis.
And truth be told, the jihadis are having no problems getting arms so why worry about leakage from our supplies?
But now that ISIS has a western front in Syria and an eastern front in Iraq, fighting ISIS creates a problem.
In Iraq, it is no problem because we should have been fighting them already.
But in Syria, fighting ISIS puts us in the position of siding with Assad. If we defeat ISIS, Assad can turn on the non-jihadis and wipe them out. This reverses the sequence of defeating Assad into one of saving Assad.
And all this because we did not act when we could when each country was a mostly discreet problem separate from each other.
But both problems got worse and bigger as we boasted of Osama bin Laden's death and the revival of General Motors, until the jihadi uprisings in each country became one big problem that makes our intervention more problematic as far as getting a good outcome in both places.
Gosh, I love our smarter foreign policy!
UPDATE: This rather fits with the problems of a joined war, no?
One day after Israeli planes struck targets in Syria, Syrian warplanes attacked targets inside Iraq late on Tuesday, a sign of the broadening conflict.