Throughout the Cold War, Bulgaria was a close ally of the Soviet Union. After the Cold War ended and the Soviet Union disintegrated, evidence emerged that Todor Zhivkov, the long-time leader of the Bulgarian Communist Party, had proposed that Bulgaria officially join the USSR as the “sixteenth Soviet republic.” However, these offers, which would have entailed the sacrifice of national sovereignty, are best understood as tactical maneuvers. Zhivkov used the tactic not just once, but at least three times. Nonetheless, it is highly unlikely that he ever intended for Bulgaria to be fully incorporated into the Soviet Union. Instead, he used Bulgarian national sovereignty as a bargaining chip during negotiations for financial and economic support to strengthen his personal rule.

During the Cold War, Bulgaria was a staunch ally of the Soviet Union, and the Bulgarian State Security (DS) service worked extremely closely with the Soviet State Security Committee (KGB) on a wide range of matters, including disinformation operations as well as “sharp measures”—abductions, sabotage, and, most notably, assassinations. Not until the Cold War ended and the DS archives in Bulgaria were made accessible were scholars able to explore these intelligence operations in great depth. Although the lack of access to the KGB's foreign intelligence collections in Yasenevo poses certain limits, the availability of DS collections, including many copies of KGB records, has been a gold mine for Western scholars of Cold War–era intelligence activities. Drawing mainly on Bulgarian archival sources, this article analyzes KGB-DS intelligence cooperation regarding disinformation and “sharp measures.” Among the topics covered are recently disclosed sources on the assassination of the dissident Bulgarian writer Georgi Markov in London 1978 and the assassination attempt against Pope John Paul II in May 1981. The article thus provides historical context for contemporary debates about Russian security services and their strategic use of disinformation and active measures.