we have been bombing them.
He doesn't seem to be aware of this. It's either that mentioning this fact would mess up the narrative that President Obama is doing nothing, or he geniunely does not pay attention to current events. And I would not be in a hurry to discount the latter notion. After all, he somehow was unaware that sectarian differences in Iraq would be a "thing", so it's possible the odd incredibly pertinent fact might get by him then and again.
But I feel comfortable in speculating that the "harm" would be in the possibility of large numbers of lost lives and destroyed infrastructure adding to the existing instability in the area, contributing to the disaffection of even more local people who would feel inclined to support any other faction, not necessarily because of hopes of good government, but because they've found another set of thugs who might be able to offer temporary protection. In the short-term sense of degrading ISIS' capabilities and possibly pushing back on the area they are able to control, sure, bombing is an option. (And I'm not opposed to those outcomes, frankly.) But let's not be glib. It won't be a long-term fix to the long-term screw-ups that resulted in a fractured and fractious region.
And that's just Iraq. Syria's government has amusingly advised Obama that the US should be very careful about unilaterally helping them with the wolf at their backdoor (and they will help! us! help! them!). Is it likely that Kristol's question has a much more complex answer than he seems to be comtemplating?
In a word: Damn. As in, people are still listening to Bill Kristol?