Yes, the rearguard is as important as the vanguard in the final analysis.
But it is not quite true to say that "[i]mperialism defeated socialism militarily in the 'Cold' War," though a number of governments and movements certainly were done in by the empire's secret agents and their local collaborators. Neither Russia nor China nor Vietnam was militarily defeated by imperialism, nor the CPI(M)-led Left Front government in West Bengal, the longest-standing elected Communist government in the world. They changed from inside.
Political struggle within a nation, within a party or between parties or both, is usually far more important than military battles between the empire and nations in the South. The empire never has enough soldiers to invade or intervene in all countries.
Michael A. Lebowitz writes that "Until workers break with the idea that capital is necessary, a state in which workers have political supremacy will act to facilitate conditions for the expanded reproduction of capital. . . . The state, accordingly, remains entirely within the bounds of the capitalist relation and is its guarantor so long as workers look upon capital's requirements as 'self-evident natural laws'" (Beyond Capital: Marx's Political Economy of the Working Class, Palgrave Macmillan, 2003, p. 191). The hitherto existing Communist Parties, in or out of state power, inside or outside the West, have more or less been parties led by intellectuals whom workers and peasants followed, so once intellectuals lose their (more or less idealistic) commitment to socialism, or once right-wing intellectuals defeat left-wing ones, the parties can easily be transformed, from above, into essentially social democratic parties, even neoliberal capitalist parties, that "act to facilitate conditions for the expanded reproduction of capital," even by bullets if that's what it takes. And they have been, some sooner than others.
Whether Maoists in Nepal, Bolivarians in Venezuela, and a few others who have not given up on socialism can break with the social democratic horizon of what can and needs to be done, when the global economy loses steam created by a credit boom, remains to be seen. Will they give in or move in, to use Lebowitz's terms? If the latter, how? -- Yoshie