The Soviet Union eventually did lose client-states historically.
Firstly Yugoslavia. The immediate Soviet response to the falling out was to plan a multiple strategic echelon invasion, war-game it, and pre-position for it. Luckily for Yugoslavia, Hungary, Romania, Bulgaria and the Soviet Union—the Korean war intervened at a juncture which prevented this invasion.
Later losses of client states occurred broadly within two contexts:
* That European client states maintain the broad economic organisation associated with the soviet-style economy, including the centrality of the nomenklatura in political and economic life.
* That European client states remain wedded into the defensive military system that ensured Soviet security.
Let us look at the key causes for intervention in Hungary and Czechoslovakia:
In Hungary workers councils supplanted nomenklatura economic, then political control; and, forced the Nagy coalition government into withdrawal from the Warsaw Pact (the precipitate cause, but after the failure of the initial intervention, the Soviet Union maintained the capacity for further intervention anyway).
In Czechoslovakia nomenklatura economic control was threatened by a variety of movements towards workers' control; and, thus, the Soviet nomenklatura's internal control over economic life in the Soviet Union was threatened.
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Soviet Intervention, incidentally, actually did fail in Hungary, resulting in a broad ceasefire and partial withdrawals of troops prior to a second intervention, which risked failure and was far too costly.
In Hungary, the easiest way to prevent the success of the second intervention is for Pal Maleter to mobilise the Honved with standing orders to repel foreign interventions into Hungarian life. This will probably increase the cost of the second Soviet Intervention above the capacity of the political committee's willingness to incur loss, if it doesn't stop the intervention directly.
The easiest way in Hungary for the second intervention to be prevented is for Nagy to emphatically refuse to leave the Warsaw Pact which should cause sufficient confusion within the political committee of the Soviet Union to delay any intervention until after either the workers councils or the geographic councils have taken control of the Honved. (My bet is on the workers' councils, the second intervention forced the Budapest councils into becoming the crux of legitimate counter power, for example, the Soviet Union directly negotiated with the Central Workers Council of Greater Budapest, rather than involving the Kadar pseudo-government in negotiations).
Czechoslovakia entirely depends on what happens in Hungary. There probably won't be an intervention into Czechoslovakia if Hungary succeeds, because either Czechoslovakia will be as repressively Stalinist as the Soviet Union or East Germany in relation to workers control, or the Soviet Union will be effectively forced into either allowing the Czechoslovak party freedom of direct in relation to economics, or the Soviet Union itself will be effectively forced into beginning to do what the Czechoslovak party tried historically to do in 1968.
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Incidentally, if two societies require "intervention" in 1956, the second society will be Poland, not Hungary. And with two, why not three?
yours,
Sam R.