Worried about possible terrorist attacks over the Christmas holiday, President Obama met on Dec. 22 with top officials of the C.I.A., F.B.I. and Department of Homeland Security, who ticked off a list of possible plots against the United States and how their agencies were working to disrupt them. In a separate White House meeting that day, Mr. Obama’s homeland security adviser, John O. Brennan, led talks on Yemen, where a stream of disturbing intelligence had suggested that Qaeda operatives were preparing for some action, perhaps a strike on an American target, on Christmas Day. Mr. Obama this month presented his...