During his first Danish posting, Gordievsky became disenchanted with his work in the KGB, particularly after the Soviet invasion of Czechoslovakia in 1968. He tried to send a covert sympathetic message to the Politiets Efterretningstjeneste (Danish Security Intelligence Service), called PET, but his three-year stint ended and he returned to Moscow before making any direct contact. By the time he arrived again in Copenhagen in October 1972 for a second three-year stint, both the PET as well as MI6 – which had been tipped off by one of Gordievsky's old university friends – felt he was a persuadable agent. MI6 subsequently made contact with Gordievsky, and began running him as a double agent in 1974.