NEW YORK — Declassified CIA files reveal that US intelligence officials went to great lengths to protect a Ukrainian fascist leader and suspected Nazi collaborator from prosecution after World War II and set him up in a New York office to wage covert war against the Soviet Union, according to a new report to Congress.

Mykola Lebed led an underground movement to undermine the Kremlin and conduct guerrilla operations for the CIA during the Cold War, said the report, prepared by two scholars under the supervision of the National Archives. It was given to Congress on Thursday and posted online....