The director of the C.I.A. said on Thursday that “potential desperation” to extract the semblance of a victory in Ukraine could tempt President Vladimir V. Putin of Russia to order the use of a tactical or low-yield nuclear weapon, publicly discussing for the first time a concern that has coursed through the White House during seven weeks of conflict. The director, William J. Burns, who served as American ambassador to Russia and is the member of the administration who has dealt most often with Mr. Putin, said the potential detonation of such a weapon — even as a warning shot...