Comparing ISIS to the US in WW2 like that is not fair. The US using nukes is not as bad as the hypothetical ISIS attack because of the starkly different moral intentions of the respective actors.
For the sake of argument, that's only true from the point of view regarding our morals and beliefs. ISIS believes what they're doing is good and right and true.
So as long as a group believes what they're doing is good, it's okay? Again, that that kind of moral equivalency is just ridiculous. Let's take a look at what ISIS believes is good and right and true:
1. Torturing and beheading non-believers and other "infidels" and broadcasting these vile acts as propaganda
2. Raping young children and forcing them into lives as sex slaves for soldiers of ISIS
3. Setting people on fire to burn them alive inside a metal cage
4. Establishing a "caliphate" where all would live under Sharia
5. Mass executions of civilians and anyone else refusing the caliphate
6. Mohammad is the Prophet, the Koran is gospel truth, and if you don't believe it, you will be executed
@lost_in_the_endless_aisle said it very well: "... the starkly different moral intentions of the respective actors" makes all the difference. Using force (including dropping bombs -- oh my!) to defend against such vicious brutality is completely legitimate and different from being a hostile aggressor who is trying to subjugate an entire section of humanity. Trying to equate that use of force with that of a brutal aggressor is just beyond the fringe of most people's belief systems. Perhaps we should give ISIS a stern talking-to instead? Maybe that would have worked with Japan too after they bombed Pearl Harbor intentionally to draw us into the war.