It may not be the assassination of Archduke Franz Ferdinand, but when historians write the definitive story of the first half of the 21st century in the U.S., a single weekend in the spring of 2011 may well go down as the era's hinge point—a 48-hour period that set into motion a string of events still unraveling now, 14 years later. That weekend, President Barack Obama authorized the mission to kill Osama bin Laden and kept it secret from the world as he delivered a cutting speech that brought down the house at the White House Correspondents' Dinner. What no...