[julieval]

Neither the category nor the clue mentioned America. The category was US Cities. I just found this:
David Ferrucci, the manager of the Watson project at IBM Research, explained during a viewing of the show on Monday morning that several of things probably confused Watson. First, the category names on Jeopardy! are tricky. The answers often do not exactly fit the category. Watson, in his training phase, learned that categories only weakly suggest the kind of answer that is expected, and, therefore, the machine downgrades their significance. The way the language was parsed provided an advantage for the humans and a disadvantage for Watson, as well. "What US city" wasn't in the question. If it had been, Watson would have given US cities much more weight as it searched for the answer. Adding to the confusion for Watson, there are cities named Toronto in the United States and the Toronto in Canada has an American League baseball team. It probably picked up those facts from the written material it has digested. Also, the machine didn't find much evidence to connect either city's airport to World War II. (Chicago was a very close second on Watson's list of possible answers.) So this is just one of those situations that's a snap for a reasonably knowledgeable human but a true brain teaser for the machine.