Q. Why is socialism in one country impossible?
A. First of all, socialism absolutely needs to be based on a high level of productivity. The lowest stage of socialism must be the highest stage of capitalism. If so many of the problems in the world today are due to the unequal distribution of resources, then the only solution is to produce more than enough and distribute it democratically in order that people have no reason to oppress one another. Nowhere in any of the writings of Marx, Engels, Lenin, or Trotsky will you find them proposing the idea of socialism in one country. The Stalinist, nationalist idea of socialism in one country has nothing to do with Marxism which has always been internationalist in perspective.
The working class has had many opportunities to carry out a socialist transformation this century, and has tried in many different countries. However only once, and then only temporarily did they succeed, in the Russian Revolution of 1917. This revolution in a backward country succeeded in overthrowing 1000 years of Tsarist autocracy, and the working class began to grapple with running the whole of society. However it was never the intention of Lenin to build socialism in one country. That is impossible, as socialism requires a massive increase in production to produce the needs of society. That requires the pooling of resources internationally. Also of course, capitalism cannot simply be defeated in a single country. The revolution must spread to other countries, and eventually the whole world.
As a result of its isolation, and its backwardness, civil war, and the assault of 21 armies of foreign intervention, the revolution in Russia hung by a thread. Without the assistance of revolutions in more economically advanced countries in Europe, there could not be socialism in Russia. If the revolutions in the rest of Europe had been successful, they could have all pooled their technology, natural resources, and populations as one in order to begin producing enough for all and spreading the revolution to the rest of the world. Instead, the isolated revolution degenerated into a bureaucratic dictatorship. The struggle for socialism must be international!