In our pessimistic age much of the Marxian left combines hopeless nationalism in its defence of unreformed welfarism and labour market rigidity, with hopeless internationalism in its insistence that only global socialism may hope to prosper.
Arguing that socialism in one country is impossible (I prefer 'social republicanism', but that's a different matter) is a handy way to dismiss the failures of the planned economy in the USSR as somehow contingent on the failure of revolution to spread internationally in the Red Years of 1918-1920. As an idea, it is most closely associated with Trotsky, and popularised I'd guess by Isaac Deutscher.
The better to differentiate himself from Stalin, Trotsky in exile more or less came to wear Stalin's charge that he had objected to the notion of building 'socialism in one country' (Sotsilizm v Odnoi Strane). In donning the caricature, Trotsky effectively adopted utopian obfuscation.
The initial Bolshevik assumption in 1917 and the few years afterwards had been that unless the revolution spread from Russia it would be crushed by intervention or blockade. This turned out not to be so: the regime survived, if only on the basis of war and Terror.
But while the Bolsheviks had hoped for immediate military and political solidarity from a spreading revolution, they never expected that international revolution would solve their domestic problems of Russian social and economic backwardness.
The idea that Russian Sovietism would have flourished economically and democratically if only Germany had fallen to Sparticism makes no sense. There was no realistic prospect for a socialist Marshall Aid Plan. International revolution in 1918 would have meant (as Trotsky and Lenin said) a Europe prostrated by civil war. There was never an option of workers in advanced countries, having thrown out their governments, cheerfully handing over huge surpluses for Russian modernisation. Socialist modernisation to be credible had to be endogenous, certainly not reliant on a deus ex machina.
Trotsky's position in the 1920s was indeed sensible (in some respects): that the USSR's development required serious engagement with world markets. He was then opposed to Stalin's autarchic notion of 'socialism in a separate country' (Sotsilizm v Otdel'noi Strane). This was not utopianism.
Trotsky's 1930s position, when he adopted the idea that socialism could not be built (not completed) in one country, was the rhetoric and metaphysics of defeat. It served as an alibi for the Bolsheviks taking power in a country that clearly did not have the requisites for a humane socialist experiment. Trotsky didn't want to acknowledge that if Stalin was wrong, the Mensheviks were right. Russia's legacy of backwardness could never have been simply wished away by international revolution.
In this regard, Trotsky's legacy is one of pipe-dreaming on the part of the post-war ultra-left: everything will be fine so long as revolution is international across much of the globe; otherwise nothing can be done. Day-dreaming replaces economics.