When did the campaign of political  repression that Mikhail Demichev was arrested during occur?
Great Purge The Great Purge or the Great Terror (Russian: Большо́й терро́р ) was a campaign of political repression in the Soviet Union which occurred from 1936 to 1938. It involved a large-scale purge of the Communist Party and government officials, repression of peasants and the Red Army leadership, widespread police surveillance, suspicion of "saboteurs", "counter-revolutionaries", imprisonment, and arbitrary executions. Mobile gas vans were invented to execute people without trial. In Russian historiography, the period of the most intense purge, 1937–1938, is called Yezhovshchina (Russian: Ежовщина ; literally, "Yezhov phenomenon", commonly translated as "times of Yezhov" or "doings of Yezhov"), after Nikolai Yezhov, the head of the Soviet secret police, the NKVD, who was later killed in the purge. It has been estimated that 600,000 people died at the hands of the Soviet government during the Purge.
Cold War (1985–91) The Cold War period of 1985–1991 began with the rise of Mikhail Gorbachev as leader of the Soviet Union. Gorbachev was a revolutionary leader for the USSR, as he was the first to promote liberalization of the political landscape (Glasnost) and capitalist elements into the economy (Perestroika); prior to this, the USSR had been strictly prohibiting liberal reform and maintained an inefficient centralized economy. The USSR, despite facing massive economic difficulties, was involved in a costly arms race with the United States under President Ronald Reagan. Regardless, the USSR began to crumble as liberal reforms proved difficult to handle and capitalist changes to the centralized economy were badly transitioned and caused major problems. After a series of revolutions in Soviet Bloc states, and a failed coup by conservative elements opposed to the ongoing reforms, in New Years Eve the Soviet Union collapsed in 1991 and the Cold War came to an end.