

Iranian children throw flowers into the sea as part of a 24th anniversary commemoration of the downing of Iran Air Flight 655. (Atta Kenare/AFP/Getty Images)

If you walked into any high school classroom in the United States and asked the students to describe their country's relationship with Iran, you'd probably hear words like "enemy" and "threat," maybe "distrust" and "nuclear." But ask them what the number 655 has to do with it, and you'd be met with silence.

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.

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.

The horrible incident brought Tehran closer to ending the war, but its effects have lingered much longer than that. "The shoot-down of Iran Air flight 655 was an accident, but that is not how it was seen in Tehran," former CIA analyst and current Brookings scholar Kenneth Pollack wrote in his 2004 history of U.S.-Iran enmity, "The Persian Puzzle." "The Iranian government assumed that the attack had been purposeful. ... Tehran convinced itself that Washington was trying to signal that the United States had decided to openly enter the war on Iraq's side."

That belief, along with Iraq's increased use of chemical weapons against Iran, led Tehran to accept a United Nations cease-fire two months later. But it also helped cement a view in Iran, still common among hard-liners in the government, that the United States is absolutely committed to the destruction of the Islamic Republic and will stop at almost nothing to accomplish this. It is, as Time's Michael Crowley points out in an important piece, one of several reasons that Iran has a hard time believing it can trust the United States to ever stop short of its complete destruction.

This is not just an issue of historical grievance: It matters in immediate geopolitical terms to the efforts by President Obama and Iranian President Hassan Rouhani to find their way to a nuclear deal and perhaps a first step toward detente. For any deal to work, both countries will have to trust that the other is sincere about its willingness to follow through on its promises. For the United States, that means trusting that Iran is really willing to give up any nuclear weapons ambitions and ramp down the program as promised (Washington has real, legitimate grounds to worry about this; Iran has its own history of misdeeds). For Iran, it means trusting that the United States will actually accept the Islamic Republic and coexist peacefully with it.

The eight-year war with Iraq, which is widely seen in Iran as a war against not just Hussein but his Western backers, and the downing of Iran Air Flight 655 that came near its conclusion, have convinced many in Iran that the United States simply cannot be trusted to let Iran be. Supreme Leader Ayatollah Ali Khamenei, Rouhani's boss, often appears to share this deep distrust. Khamenei and other hard-liners could scuttle any deal; a similar drama will likely play out in Washington.

If Iran believes that the United States is so committed to its destruction that it would willingly shoot down a plane full of Iranian civilians, then Tehran has every incentive to assume we're lying in negotiations. It also has strong incentives to try to build a nuclear weapon, or at least get close enough to deter the American invasion that it feared was coming in 1988 and perhaps again in 2002 with President George W. Bush's "axis of evil" speech.

Americans might not know about Flight 655. But Iranians surely do -- they can hardly forget about it.