James Taranto links to this story from USA Today on anti-American unpleasantness in Europe. It does happen, though it's not incredibly common. It's a good idea not to talk about politics or about the United States in general with Spaniards unless you're ready for an argument; don't bring up either subject yourself, ever. It's not too common, though, for people to be out-and-out rude if you're well-behaved and friendly. If someone starts in, just say, "I respect everyone's opinion, and I'm willing to listen to yours, but I'd prefer to talk about something else." Most Europeans are basically good folks like you and me and this will bring your interlocutor to his senses and make him realize that he shouldn't take out his frustrations on some innocent tourist. Then talk about how nice Barcelona is or whatever.