Try the same thing in an Iranian classroom, asking about the United States, and you'd probably hear some of the same words. Mention the number 655, though, it's a safe bet that at least a few of the students would immediately know what you were talking about.

The number, 655, is a flight number: Iran Air 655. If you've never heard of it, you're far from alone. But you should know the story if you want to better understand why the United States and Iran so badly distrust one another and why it will be so difficult to strike a nuclear deal, as they're attempting to do at a summit in Switzerland this week.

The story of Iran Air 655 begins, like so much of the U.S.-Iran struggle, with the 1979 Islamic revolution. When Iraq invaded Iran the following year, the United States supported Iraqi leader Saddam Hussein against the two countries' mutual Iranian enemy. The war dragged on for eight awful years, claiming perhaps a million lives.