This question has come up in a number of threads and I thought it might be useful to start a specific thread to discuss it.
Trotskyites assert that it was doomed from the start because it is impossible to "build socialism in one country" whereas the traditional Marxist-Leninist view is that a clique of revisionists around Khruschev simply came to power after the death of Stalin and set about restoring capitalism.
What made me return to this question is that by chance I came across an interesting article by someone called Ted Talbot who does not support either position and has an alternate view.
Talbot points out that changes in the Soviet Union and Eastern Europe were not brought about by economic collapse or by popular revolution from below. The changes were in fact instigated by the elite running these countries.
In this regard he quotes from an article by Kotz and Weir:
So this poses the question .. how was it that 60 years after the revoultion you ended up with a ruling elite running the show who had more affinity with capitalism than with socialism never mind communism ... and who saw the introduction of a straightforward market economy as the best way to enrich themsleves.Conventional wisdom tells us that the remarkable demise of the Soviet Union in 1991 was propelled by the collapse of its socialist economy, leading the citizenry to peacefully sweep aside the nation's Communist leadership and their misbegotten socialist system. Yet, if one inquires into the whereabouts of the allegedly deposed Communist leaders, one finds most of them not languishing in exile, but still in high-level positions in the 15 new nations that emerged from the USSR.
Furthermore, most of them are a great deal richer than than they were before the Soviet Union's demise. Two years after this odd revolution, 11 of these 15 new nations were headed by former top Communists.
In contrast to the conventional wisdom, the Soviet revolution of 1991 was made, not against the small elite that ran the Soviet Union, but rather by that elite. And it was not a collapse of the USSR's planned economy that drove this process, because no such collapse took place. While the Soviet planned economy encountered serious problems after the mid-1970s, it was far from collapsing at the end of the 1980s. Rather, the Soviet elite dismantled their own system in pursuit of personal enrichment.
It is this question that Talbot attempts to answer.