Question: By approximately what percentage did the Black Death reduce the population of the region that the Soviet Union feared invasion from?

Knowledge 1: Warsaw Pact
The strategy behind the formation of the Warsaw Pact was driven by the desire of the Soviet Union to dominate Central and Eastern Europe. This policy was driven by ideological and geostrategic reasons. Ideologically, the Soviet Union arrogated the right to define socialism and communism and act as the leader of the global socialist movement. A corollary to this idea was the necessity of intervention if a country appeared to be violating core socialist ideas and Communist Party functions, which was explicitly stated in the Brezhnev Doctrine. Geostrategic principles also drove the Soviet Union to prevent invasion of its territory by Western European powers. [SEP] Knowledge 2: Black Death
The most widely accepted estimate for the Middle East, including Iraq, Iran and Syria, during this time, is for a death rate of about a third. The Black Death killed about 40% of Egypt's population. Half of Paris's population of 100,000 people died. In Italy, the population of Florence was reduced from 110–120 thousand inhabitants in 1338 down to 50 thousand in 1351. At least 60% of the population of Hamburg and Bremen perished, and a similar percentage of Londoners may have died from the disease as well. Interestingly while contemporary reports account of mass burial pits being created in response to the large numbers of dead, recent scientific investigations of a burial pit in Central London found well-preserved individuals to be buried in isolated, evenly spaced graves, suggesting at least some pre-planning and Christian burials at this time. Before 1350, there were about 170,000 settlements in Germany, and this was reduced by nearly 40,000 by 1450. In 1348, the plague spread so rapidly that before any physicians or government authorities had time to reflect upon its origins, about a third of the European population had already perished. In crowded cities, it was not uncommon for as much as 50% of the population to die. The disease bypassed some areas, and the most isolated areas were less vulnerable to contagion. Monks and priests were especially hard hit since they cared for victims of the Black Death.
Knowledge 1: Warsaw Pact
The strategy behind the formation of the Warsaw Pact was driven by the desire of the Soviet Union to dominate Central and Eastern Europe. This policy was driven by ideological and geostrategic reasons. Ideologically, the Soviet Union arrogated the right to define socialism and communism and act as the leader of the global socialist movement. A corollary to this idea was the necessity of intervention if a country appeared to be violating core socialist ideas and Communist Party functions, which was explicitly stated in the Brezhnev Doctrine. Geostrategic principles also drove the Soviet Union to prevent invasion of its territory by Western European powers. [SEP] Knowledge 2: Consequences of the Black Death
Consequences of the Black Death included a series of religious, social, and economic upheavals, which had profound effects on the course of European history. The Black Death was one of the most devastating pandemics in human history, peaking in Europe between 1347 and 1350 with 30% to 65% of the population killed. It reduced world population from an estimated 450 million to between 350 and 375 million. It took 80 and in some areas more than 150 years for Europe's population to recover.
Knowledge 1: Warsaw Pact
The strategy behind the formation of the Warsaw Pact was driven by the desire of the Soviet Union to dominate Central and Eastern Europe. This policy was driven by ideological and geostrategic reasons. Ideologically, the Soviet Union arrogated the right to define socialism and communism and act as the leader of the global socialist movement. A corollary to this idea was the necessity of intervention if a country appeared to be violating core socialist ideas and Communist Party functions, which was explicitly stated in the Brezhnev Doctrine. Geostrategic principles also drove the Soviet Union to prevent invasion of its territory by Western European powers. [SEP] Knowledge 2: Black Death
The Black Death, also known as the Great Plague, the Black Plague, or simply the Plague, was one of the most devastating pandemics in human history, resulting in the deaths of an estimated 75 to 200 million people in Eurasia and peaking in Europe from 1347 to 1351. The bacterium Yersinia pestis, which results in several forms of plague, is believed to have been the cause. The plague created a series of religious, social and economic upheavals, which had profound effects on the course of European history.

Answer: 30% to 65%