that night, with Col. Muammar el-Qaddafi’s forces turning back the rebellion that threatened his rule, Mrs. Clinton changed course, forming an unlikely alliance with a handful of top administration aides who had been arguing for intervention.

Within hours, Mrs. Clinton and the aides had convinced Mr. Obama that the United States had to act, and the president ordered up military plans, which Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, hand-delivered to the White House the next day.