The day before the Obama administration was due to slap new sanctions on Iran late last month, Iranian Foreign Minister Javad Zarif warned U.S. Secretary of State John Kerry the move could derail a prisoner deal the two sides had been negotiating in secret for months. Kerry and other top aides to President Barack Obama, who was vacationing in Hawaii, convened a series of conference calls and concluded they could not risk losing the chance to free Americans held by Tehran. At the last minute, the Obama administration officials decided to delay a package of limited and targeted sanctions intended...