Bulgaria–United States relations to work. Bulgaria became a member of the Warsaw Pact on May 14, 1955. Though no Soviet troops were stationed on Bulgarian soil, in 1968, Zhivkov sent a Bulgarian division to join Polish, Hungarian and Soviet troops to crush a popular uprising in Prague. In 1978, the Bulgarian secret police were implicated in the assassination in London of Georgi Markov, a Bulgarian dissident who was a correspondent for the BBC World Service, Deutsche Welle radio and the U.S. sponsored Radio Free Europe. Markov was assassinated with a poisoned umbrella on September 7, Zhivkov's birthday. The rise of democratic movements across
Bulgarian umbrella Vladimir Kostov the same year in the Paris "Métro". The poison used in both cases was ricin. Both assassination attempts are believed to have been organized by the Bulgarian Secret Service of the time of the Cold War with the assistance of the KGB. Such an umbrella was intended to be used in the assassination of Pallo Jordan and Ronnie Kasrils by the South African Civil Cooperation Bureau death squad. These two cases inspired the 1980 French film "Le Coup du parapluie" ("The Umbrella Coup") directed by Gérard Oury and starring Pierre Richard, and Bulgarian writer Stefan Kisyov's novel "The