Fighting ISIS directly implies that they will line up in a field wearing, say, white. We can line up wearing blue and march at them.
In reality, I'd expect the consequences of taking them on 'directly' to be mass civilian casualties, the enemy melting away and a new generation feeling their kin have been wronged and require avenging actions.
It thoroughly amazes me that so many people constantly think we can win a war crisply and definitively and neatly put an end to 'that' issue. Whatever 'that' issue is at any point in history.