'What is Stalinism' asks Charles, and 'what is the difference between Leninism and Stalinism'? I think there is a clearly stated break, which is this:
‘in the new period … the old formula … becomes incorrect and must inevitably be replaced by another formula, one that affirms the possibility of the victory of socialism in one country.’ 'The Social-Democratic Deviation in our party', Report delivered at the Fifteenth all-union conference of the CPSU(B), Nov 1, 1926 in Stalin On the Opposition (Foreign Language Press, Peking 1974)
Stalin was aware that his was a new policy. The policy was an attempt to deal with the stabilisation of capitalism post-world war I, under the Dawes plan. The policy was pragmatic - an attempt to retrench - an not wrong for that. It was though, delusional, in that there was no possibility of building socialism in Russia, without access to western technology, on terms that simply were not on offer.
The negative consequences of the 'socialism in one country' policy were that European communist parties were tempted to mix up diplomatic advantage for the USSR with what was best for building revolution in their own countries. At key moments, the advice from Moscow was to put off the revolution, where it jeopardised the Soviet Union's own diplomacy. At that point, you could say that 'Stalinism' was a reactionary influence. In the USSR the police measures that the bureaucracy took to shore up the bad policy were extreme, and destructive.
^^^^ CB: First, there is more to Leninism than the issue of building socialism in one country. But even more, I'll have to look ,but I think Lenin indicated it was possible to build socialism in one country.
But my main response would be to agree with your point of pragmatism; that once it was clear that there would not likely be a socialist revolution in one of the advanced capitalist countries - Germany , France - the Soviet CP would have to basically "resign" or "liquidate" the Soviet state and revolution is they didn't claim that it was possible to build socialism in one country; as theirs was the only country building socialism. I doubt Stalin was delusional. More like fibbing. And waiting and hoping a big capitalist country might have a revolution. Also, crash industrialization was based on awareness of the vulnerability of socialism in a semi-advanced country in a world of powerful imperialist countries. On the other hand, the Soviet Union lasted for a long time, and it had significant socialist features.
Then there's a lot of pragmatism or realism in Leninism, so I'm not sure that this distinguishes the CP led by Stalin from that led by Lenin; and as I said , Lenin may have said some things on building socialism in one country for a while.
On "the word from Moscow" , I'm thinking if the Soviet Communist Party publically urged revolutions in other countries, those revolutions could be countered based on the claim of outside agitators and basically, nationalism.