In a reply to his Social Democratic opponents, who claimed that Marx's theory could only be realized in the developed capitalist countries, Lenin wrote in 1920: Russia achieved Marxism, the only correct revolutionary theory, by truly suffering through a half-century's history of unheard-of torments and sacrifices, unprecedented revolutionary heroism, incredible energy, and selfless seeking, learning, testing in practice, being disappointed, verifying, and making comparisons with the experience of Europe.