When did the USSR turn its back on Marxism?

On a discussion list I presented a mainstream of Marxist development this way: "Marx and Engels in the 1840s through the Paris Commune to the October Revolution in 1917 and on to the Left Opposition and Fourth International - it's Bolshevik-Leninist built on rock-solid Marxist (and that's Marx and Engels) foundations".

DV asked me: "So at what point does your allegiance split off (or maintain the true centre) of that line, later in the SU? And why?

And I replied:

The change in the political character of the USSR can be located most simply in the year 1924. Lenin died in January, 1924, and during the year Stalin was able to consolidate his power in the Bolshevik party apparatus, principally by opening the floodgates to new members with interests that were bureaucratic and careerist rather than class-based and socialist. The struggle against this development can be traced in the history of the Left Opposition to Stalin and the bureaucracy, which Trotsky founded in 1923 while Lenin was still alive, albeit incapacitated.

Evidence of healthy working-class socialist internationalism is abundant in the records of the first four Congresses of the Third International (ie before 1924). After 1924 the politics of the Soviet government turn against  all the principles of the Bolshevik party that carried out the October Revolution and led the successful defence of the new non-capitalist workers state against the imperialists and reactionaries of the surrounding world and Old Russia.

The best brief account of the general development can be found in Trotsky's book The Revolution Betrayed (1936)  (http://www.marxists.org/archive/trotsky/1936/revbet/index.htm)

The Stalinist bureaucracy instituted a new regime in the Soviet Union that Trotsky and the Left Opposition characterized as politically counter-revolutionary, although on a non-capitalist economic foundation. In other words, the USSR remained a workers state, but was run by a degenerate and counter-revolutionary regime

The catastrophe of the Nazi capture of power in Germany in 1933, resulting largely from the appalling anti-Marxist leadership provided by the German Communist party following orders from Moscow - Social-Democracy was called the main enemy of the German working class, rather than Nazism - drove Trotsky and his comrades to conclude that the Third International was beyond recovery and led them to prepare the foundation of a new working-class socialist international - the Fourth International. This was achieved in 1938, and the founding document - the Transitional Programme, "The Death Agony of Capitalism" (http://www.marxists.org/archive/trotsky/1938/tp/index.htm)- is the best short presentation of modern Marxist perspectives in the imperialist world after the victories of Stalin and Hitler. That is in a world dominated by counter-revolutionary regimes from the US to the USSR. At the same time as the world was split into two antagonistic economic camps - capitalist-imperialist in the US and Europe, and non-capitalist  (what we could call proto-socialist) in the Soviet Union.