Here is a nice rundown of Paul Krugman's work in his own words.
It is interesting to note that where industries tend to concentrate is, oftentimes, a matter of historical accident. But you wonder if there isn't at least a bit more to the story. He talks of wide-body aircraft-- I couldn't help but to think that Boeing produced in Seattle for a long while, and still does the majority of its factory production there, but it up and moved its corporate office to Chicago not too long ago. Big, and costly at least in transactions terms, moves like that can't be chalked up to historical accident. The location in Seattle in the first place might a historical accident-- but maybe it's not. I'd be willing to bet there's a political economy story to the location of a lot of large manufacturers. An "accident" in the trade sense is far from that in the economic history/political economy sense.
Still, the overarching moral is that it's not particularly important that the jets are produced Seattle, or Chicago, or Mexico City, or Tokyo, but that they are produced and everyone trades for them-- places that we wouldn't expect to trade according to existing trade theory. And yes, that's obvious, and that's how you know it's a profound finding.