This is a slipper slope if I've ever seen one. Aid to Syrian rebels is already known, one of the parties to which a proxy war might be a somewhat reasonable assumption. However, given the long involvement of the US in Iraq, this is a very far-fetched conspiracy theory to say the least.
A lot of these guys ended up in ISIS, it would be foolish to assume otherwise. If you view America's foreign policy aim as maintaining an unstable middle east, then it's not a slippery slope argument.
