"Even supposing for a moment that owing to unfavourable circumstances and hostile blows the Soviet regime should be temporarily overthrown, the inexpungable impress of the October revolution would nevertheless remain upon the whole future development of mankind." Leon Trotsky, 1931, The Soviet Union must now be understood as a completed historical process. This history begins with the historical origins of the Soviet Union, traces through its history both internally and in relation to the rest of the world, and explains how it ceased to exist in 1991-93. The social and historical roots of the two world wars are dealt with in depth. Finally, some lessons are learned from the whole experience.