He was a carefree Italian with a recent law degree from a Roman university. She was “a totally Virginia girl,” as she puts it, raised across the road from George Washington’s home. Their romance, sparked by a 2006 meeting in a supermarket in Rome, soon brought the Italian, Domenico Salerno, on frequent visits to Alexandria, Va., where he was welcomed like a favorite son by the parents and neighbors of his girlfriend, Caitlin Cooper.

But on April 29, when Mr. Salerno, 35, presented his passport at Washington Dulles International Airport, a Customs and Border Protection agent refused to let him into the United States. And after hours of questioning, agents would not let him travel back to Rome, either; over his protests in fractured English, he said, they insisted that he had expressed a fear of returning to Italy and had asked for asylum.

Ms. Cooper, 23, who had promised to show her boyfriend another side of her country on this visit  meaning Las Vegas and the Grand Canyon  eventually learned that he had been sent in shackles to a rural Virginia jail. And there he remained for more than 10 days, locked up without charges or legal recourse while Ms. Cooper, her parents and their well-connected neighbors tried everything to get him out.

Mr. Salerno’s case may be extreme, but it underscores the real but little-known dangers that many travelers from Europe and other first-world nations face when they arrive in the United States  problems that can startle Americans as much as their foreign visitors.