>Why did so many of the movements of national liberation end so badly?
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My short answer is these movements were all swamped by the weight of the
still-expanding US-led global capitalist economy. Classical Marxists
expected the advanced capitalist countries to contract and collapse in the
20th century, but history took a different course, which decided the fate of
the social movements in the less developed countries. We may simply be
living through an historical detour in the citidels of capitalism, but if so
it is a much longer detour than Marx, Engels, and their pre-WWI followers
would have predicted.
Smaller countries, especially in Africa and Central America which tried to embark on an independent path of development were too weak to resist imperialist subversion and to complete their national wars of liberation. The larger ones whose liberation movements did succeed under Marxist leadership - China, Vietnam, the the fSU - ultimately found that they needed to integrate into the capitalist world market, which required abandoning their planned and closed economies.
These movements, though, did accomplish their so-called "democratic tasks" - destroying the landlord class, industrialization, and raising the living and cultural standards of their populations. They were just never able to go beyond that. Cuba is the only remaining economy where private ownership is suppressed, although it too has had to make accomodations to it, and everyone will be watching to see how matters develop after the revolutionary generation represented by Castro passes from the scene. The Islamists in Iran and the ANC in South Africa were nominally "anti-imperialist", and although they are far from the junior partners of imperialism as their predecessor regimes were, they have also to adapt to the pressures of world capitalism, and in neither case have the results been what the left hoped for.
For there to have been a happier ending, these societies would have had to attain a higher level of productivity than the advanced capitalist countries. The Soviet and Chinese revolutions elevated the living standards of the masses, but younger post-revolutionary generations took these for granted and measured the success of their own system, not in relation to the absolute deprivation of the past, but against the more productive, wealthier, and less restrictive capitalist societies, whose material success they wanted to emulate. I doubt ideologies less compromised than "Stalinism" could have overcome this productivity gap through popular education and agitation.
If the effort to understand why things turned out differently makes me appear right wing to some, well so be it.