YAROSLAVL, Russia — The mayor of Burlington, Vt., wrote to a Soviet counterpart in a provincial city that he wanted the United States and the Soviet Union to “live together as friends.”

Unbeknown to him, his desire for friendship meshed with the efforts of Soviet officials in Moscow to “reveal American imperialism as the main source of the danger of war.”

That mayor was Bernie Sanders, and the story of his 1988 trip to the Soviet Union has been told before. But many of the details of Mr. Sanders’s Cold War diplomacy before and after that visit — and the Soviet effort to exploit Mr. Sanders’s antiwar agenda for their own propaganda purposes — have largely remained out of sight.

The New York Times examined 89 pages of letters, telegrams and internal Soviet government documents revealing in far greater detail the extent of Mr. Sanders’s personal effort to establish ties between his city and a country many Americans then still considered an enemy despite the reforms being initiated at the time under Mikhail S. Gorbachev, the Soviet general secretary.

They also show how the Kremlin viewed these sister city relationships as vehicles to sway American public opinion about the Soviet Union.

“One of the most useful channels, in practice, for actively carrying out information-propaganda efforts has proved to be sister-city contact,” a Soviet Foreign Ministry document provided to Yaroslavl officials said.

The documents are part of a government archive in Yaroslavl, Russia, which became the sister city of Burlington. The files are open to the public, though archivists there said that, until now, no one had asked to see them.