Brezhnev Doctrine
Moscow's declared right to invade any socialist country whose socialism it judged to be in danger, announced after Czechoslovakia in 1968 and abandoned in 1989.
The Brezhnev Doctrine was the Soviet Union's answer to an awkward question: what does sovereignty mean inside a bloc where every member is supposedly a free and equal socialist state, and one of them keeps trying to leave? The answer, formalized in 1968, was that it means very little.
The occasion was Czechoslovakia. Under Alexander Dubcek the Czechoslovak party had spent the spring of 1968 liberalizing, abolishing censorship, tolerating debate, promising what it called socialism with a human face. It was not leaving the Warsaw Pact and it insisted repeatedly that it was not abandoning communism. Moscow decided that was not the point. On the night of August 20, 1968, roughly half a million Warsaw Pact troops crossed the borders and the Prague Spring was over.
Stating the rule out loud
What made this different from the crushing of Hungary in 1956 was that this time the Soviets wrote down the principle. An article in Pravda that September, under the name of Sergei Kovalev, argued that the sovereignty of an individual socialist state could not be set against the interests of world socialism, and that when internal or external forces threatened socialism in one country it became the concern and the duty of all the others. Brezhnev spelled it out in a speech to the Polish party congress that November.
Western commentators named it the Brezhnev Doctrine and summarized it as limited sovereignty: a country was free to run its own affairs right up to the moment Moscow judged its socialism at risk, and Moscow was the sole judge.
Consequences
The doctrine did its job as a deterrent. For the next two decades, no Eastern European leadership seriously tested it. Polish reformers in 1980 and 1981, facing the mass movement of Solidarity, operated under the constant calculation of whether Soviet tanks would come, and the Polish government imposed martial law itself in part to preempt them.
It also cost Moscow dearly abroad. Communist parties in Western Europe, particularly the Italians, broke publicly with the Soviets over Czechoslovakia and began the drift toward Eurocommunism. In China, already estranged, the doctrine confirmed the view that the Soviet Union was an imperial power in socialist clothing, and Beijing spent the following years fortifying its northern border. And in 1979, when Soviet forces entered Afghanistan to replace a government they had lost confidence in, the doctrine supplied the justification.
The Sinatra Doctrine
What killed it was that it was never sustainable without the willingness to use it, and by the late 1980s Mikhail Gorbachev was not willing. He told Eastern European leaders privately, from 1988 onward, that Soviet troops would not save them, and most of them did not believe him. In 1989 they found out he meant it: Poland's partly free elections, Hungary opening its border, the fall of the Berlin Wall in November, the Czechoslovak Velvet Revolution, all of it unfolded without a single Soviet division moving.
In October 1989 the Soviet foreign ministry spokesman Gennadi Gerasimov was asked on American television what had replaced the Brezhnev Doctrine. He said the Sinatra Doctrine, because each country could now do it their way. The joke went around the world, and it was as close to an official obituary as the policy got.
The episode is a lesson in how quickly an empire held together by expectation can dissolve. Nothing changed militarily in 1989; the Soviet garrisons were still there. What changed was that everyone concluded they would not be used, and once that belief went, six governments fell in a single autumn.