U.S. used to rally behind a president when bad things happened abroad. Not any more. What happened in Tehran on Nov. 4, 1979, was an American president’s worst nightmare — especially when that president already had an approval rating hovering at just 30%, thanks to a lousy economy. An angry mob riled by that year’s Islamist revolution overran the U.S. Embassy in the Iranian capital, and by nightfall more than 50 Americans had been taken hostage. For a nation still reeling from its failure in the Vietnam War that ended just four years earlier, the Iranian hostage crisis made the...