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. Toward the end of the war, on July 3, 1988, a U.S. Navy ship called the Vincennes was exchanging fire with small Iranian ships in the Persian Gulf. The U.S. Navy kept ships there, and still does, to protect oil trade routes. As the American and Iranian ships skirmished, Iran Air Flight 655 took off from nearby Bandar Abbas International Airport, bound for Dubai. The airport was used by both civilian and military aircraft. The Vincennes mistook the lumbering Airbus A300 civilian airliner for a much smaller and faster F-14 fighter jet, perhaps in the heat of battle or perhaps because the flight allegedly did not identify itself. It fired two surface-to-air missiles, killing all 290 passengers and crew members on board.