Well Putin isn't doing anything to us, so why would we go after him? I'll assume that Putin isn't a good example, so let's try Osama bin Laden. If we take out OBL (which we did), what's to stop the rest of Al-Qaeda from going after Obama? Well, nothing...in fact they seem to be trying to do just that. Where does it stop? When the war on terror ends. I know, for a long time it looked like there would be no end to the war on terror, but with the onset of the Arab Spring there's hope for Middle Easterners to join a normal political process rather than turning to radicalization. And we see that in how Al-Qaeda had no appeal whatsoever to the protesters in Egypt, so the hope is that radical groups will eventually fade away and die out.