EXCLUSIVE: U.S. special operations forces mounted an unsuccessful mission to rescue two American University of Afghanistan professors kidnapped in Kabul last month -- after an earlier mission was aborted when the White House withheld its approval -- defense officials with knowledge of the incident told Fox News. Fox News is told the operation, which took place a few days after their Aug. 7 kidnapping, killed seven enemy fighters. But when the firefight ended, there was no sign of the hostages. One of the hostages is American; the other is Australian. It was not the first attempt by the U.S. military...