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
Wood Green ricin plot incident; they all survived. All other suspects were either released without charge, acquitted, or had their trials abandoned. Physicians throughout the United Kingdom were warned to watch for signs that patients had been poisoned by ricin, and the public health director for London urged the public not to be alarmed following some media reports. Britain's largest circulation tabloid newspaper, "The Sun", reported the discovery of a "factory of death", and other newspapers warned on their front pages "250,000 of us could have died", "Poison gang on the loose" and "Killer with no antidote". The fact that no ricin had been