Once again, Restating the Obvious has to provide a fresh serving of obvious because (so far as I can find) nobody else is willing to say it.
One of the reasons that ISIL is taken more seriously than other, equally obnoxious Islamic cults is that is has a lot of money (relatively speaking; it has nothing compared with its alleged opponent, the mostly imaginary "state" of Iraq). Money makes the mare go, in insurgencies as in other endeavors. England defeated Napoleon largely by shipping gold to Spaniards who were willing to die for their country but not to starve for it.
The United States pulled off the same coup against Russia in Afghanistan. It is the greatest thing about being rich: you can get people to do your dying for you.
But it is not a mere financial transaction. There have to be patriots on the other side. No country ever spent more money trying to get brown people to die for it than the United States did in Vietnam, but no one wanted to die for Madame Thieu's racehorses. Money could not make those mares go.
I am sure that money given to alleged opponents of ISIL will earn a poor return, except in Kurdistan, because although "our" browns have something to fight against, they have nothing to fight for. (This explains the phenomenon of switching sides that is the most obvious characteristic of politics in that part of the world. People who judged the "surge" and Awakening to have been a success were deluded, and this is the reason that, although the surge had the desired short term effect of allowing Bush to skedaddle out of his lost war, it had no lasting effect.)
But just because the positive use of money is not really available in southwest Asia to the West, thaet does not mean that the negative use is not available. The United States cannot buy allies, but it has the capacity to deny money to ISIL.
Not all of it, but apparently the largest source is selling oil from its captured oil fields. That can be choked off, easily.
The American government has been making some effort in this direction, using purely financial methods -- trying to shut off transfers of the income from the oil sales. This effort has failed.
There is a simpler way. It will not be used, because it is equally horrifying to leftists and rightwingers. So horrifying, so unthinkable that it has, in fact, not been thought of.