Via Greg Hengler and Guy Benson, if this answer doesn’t destroy him, nothing will. I get the sense watching it that he’s so unsure of how to answer this exceedingly easy question that he defaults to Netanyahu’s position on the assumption that prisoner swaps must always be the wise, statesmanlike, conservative thing to do. The alternative, that he knows what he’s talking about yet is still sincerely inclined to release the guy who planned 9/11 plus dozens upon dozens of other jihadi fanatics in exchange for one G.I., is even worse. I’d bet 95 percent of people asked on the street could answer this correctly, yet somehow our frontrunner not only blows it but feels obliged to hedge weakly by noting that he’d need all the facts to make a proper decision. Here’s a fact: Khaled Sheikh Mohammed blew up the World Trade Center. What other facts do you need?

And to think, I thought this would be his worst foreign-policy answer of the week:

Halfway through the question about the electric fence, Cain butted in with “it was a joke!”

“Let me first say it was a joke, and some people don’t think that it was a good joke, and it’s probably not a joke that you’re supposed to make if you’re a presidential candidate,” Cain continued. “I apologize if it offended anyone. Mea culpa, mea culpa, mea culpa.”

Only, it might not be a joke, he later said, before finally saying he just doesn’t want to offend anyone.

“I don’t like to offend anyone…however, I don’t apologize for using a combination of a fence. And it might be electrified — I’m not walking away from that,” Cain backtracked. “I just don’t want to offend anybody. It was a joke to the extent in the context of the views of that speech, but in terms of what we need to do, I fully intend to do so because I’m more sensitive to our citizens being hurt.”

Ace flags a few other giant gaps in Cain’s foreign-policy knowledge and asks, “Has ‘homework’ and ‘thinking things through, in advance’ become ‘un-conservative’ over the past year?” When I hear him give answers like the one he gives below about Gitmo, it makes me wonder why he wants to be president in the first place. If you’re this disengaged from one of the executive’s core duties, why would you want those duties at all? It’d be like me trying to get hired at an accounting firm because I think I can “make a difference” even though I know zip zero zilch about accounting. Normally I hate pop-quiz foreign policy questions that candidates get like “Who’s the prime minister of Nigeria?”, but after all these gaffes I’m thinking the media’s well within its rights at this point to test Cain on basic stuff. And I don’t mean who the president of Ubeki-beki-beki-beki-stan-stan is. I mean stuff like, “Is Iran a Sunni or Shiite country, and why does that matter?” Anyone who’s read the news intermittently over the past 10 years should be able to offer a basic answer to that one. I’d be curious to hear Cain’s.

This isn’t his only new problem, either. A study by the Tax Policy Center that’s out tonight claims his 9-9-9 plan would raise taxes on 84 percent of U.S. households. If you don’t trust their analysis, try Ramesh Ponnuru’s skeptical take on Cain’s plan in Bloomberg today or Grover Norquist’s misgivings about it when talking to ABC yesterday. In fact, Stephen Moore, who helped Cain devise the plan, has already started to walk back part of it, saying that the sales tax aspect should be dropped in favor of a nine-percent payroll tax instead. Exit question: Have we seen Cain’s high-water mark now come and go?

Update: I didn’t see it happen live, but Guy Benson tells me that Cain was interviewed again by Blitzer shortly after the debate and said he “misspoke” in his Gitmo answer. We’ll have to wait for the clip tomorrow for his full response.