If there’s a war between the United States and Iran, it may well start on the water. After all, it’s happened before. Twenty years ago American ships were under fire in the Persian Gulf, and mines laid by the mullahs’ men nearly sank a U.S. guided missile cruiser. In April 1988 the American and Iranian navies fought the biggest air-sea battle waged since World War II. By the time it was over, carrier-based U.S. attack planes had sunk the frigate Sahand and disabled the frigate Sabalan, the pride of the Iranian navy. That’s why the comment by Defense Secretary Robert Gates on Tuesday about the brief deployment of a second U.S. aircraft carrier to the gulf was so terse and so telling. “I don’t see it as an escalation,” Gates said. “I think it could be seen, though, as a reminder.”