A cache of a quarter-million confidential American diplomatic cables, most of them from the past three years, provides an unprecedented look at back-room bargaining by embassies around the world, brutally candid views of foreign leaders and frank assessments of nuclear and terrorist threats.

Some of the cables, made available to The New York Times and several other news organizations, were written as recently as late February, revealing the Obama administration’s exchanges over crises and conflicts. The material was originally obtained by WikiLeaks, an organization devoted to revealing secret documents. WikiLeaks posted 220 cables, some redacted to protect diplomatic sources, in the first installment of the archive on its Web site on Sunday.