I'm working on a real blog post, but in the meantime, Ezra Klein has a good post:
[McCain's] statement, in other words, made no sense. It was a war against Arabs, and maybe some Persians. not a limited conflict against al Qaeda. As Obama says, one of the clear distinctions between the Left's approach to terrorism and the Right's approach to terrorism is that the Left wants to limit the scope of the conflict, while the Right wants to expand it. So though it was only al Qaeda who attacked us on 9/11, Romney and Giuliani and McCain and plenty of their colleagues want to zoom out from al Qaeda to terrorism, and from terrorism to Islamic extremism. Rather than this being an effort to hunt down al Qaeda, it becomes a war to hunt down al Qaeda, destroy Hezbollah, eradicate Hamas, overthrow Saddam Hussein, change the regime in Tehran, crush the Muslim Brotherhood, and confront Syria, and whatever else Bill Kristol thought of while eating his Cheerios that week. It is an incredibly dangerous and incoherent approach. And it marks a genuine difference between Obama and McCain.
Of course, acknowleging that those brown people over there aren't a monolithic block would require having a "nuanced" view of the world, something Republicans aren't too keen on.