The Russian threat|
The second half of the twentieth century has been dominated by the Cold War. The reason for having the Cold War was the threat posed by the Soviet Union. To almost all accounts the Cold War started with the speech in 1948 by Churchill announcing the existence of an Iron Curtain in Europe, thereby describing the boundary between the countries occupied by the Western allies, and those by Russia. In actual, there had nothing been going on in terms of hostilities, the argument was about how the occupied countries were ruled. Of course, both occupying powers tries to impose there own system of government. So the start of the Cold War was in effect an ideological war.
It is difficult to interest a civilian population into an ideological argument for longer time. The Cold War was maintained by turning the ideological threat into a physical, that is suggesting that the Soviet Union had plans to invade Western Europe: The Russians are coming. People were given instructions on what to do in case of a nuclear attack: what to do when the bomb falls. This fear dominated much of the fifties and sixties in Western Europe, and in the United States even longer.
One of the arguments to underline the danger was the assertion that invasions of Western Europe from the east had been common in history. Let us name a few of the mutual incursion in east and west. In 1803 the invasion of Russia by the French led by Napoleon. In 1830 the invasion of the Crimea, Russia, by the English. In 1915 the invasion of Russia by the Germans. In 1918 the small scale interference in the Russian civil war by the English. In 1941 the invasion of Russia by the Germans. Summing up one comes to the conclusion that if there was any justified fear, it was a fear of the Russians from an invasion of the west. One might argue that this very real fear is an argument for the enduring occupation of the countries of Eastern Europe, in order to form a buffer for a new invasion.
The original question is: was there ever any credible threat from the Soviet Union. Since the fall of the Soviet Union, now more than a decade ago, much has become known about the things that happened over there. However, until now there has not emerged any indication of a plan to attack Western Europe, and one can safely assume there never was one.