Marx thought that he had found a “scientific” basis for socialism by seeing all important social activity as shaped by the means of production and means of exchange in the culture under study. He had many brilliant insights, and his method of analysis can still be useful today; but it seems clear that capitalist nations and their populations are often swept up in nationalistic, religious, or other manias which override their economic self-interest. The Underground Man observes in Notes from Underground that people often act against their own best interests–and that seems to be as true of nations as it is of individuals.
During the Vietnam War radicals often quoted a statement of President Eisenhower’s that Southeast Asia should not be “lost” to Communism because of its valuable natural resources. But anyone studying the Vietnam war would be hard-pressed to find evidence of any economic benefit accruing to the U.S. from it. Indeed, Eisenhower probably felt compelled to justify what was essentially an ideological battle in capitalist terms because such attitudes are considered “rational” in capitalist nations.
A more sophisticated Marxist analysis argues that the domination of world economies and the creation of neocolonialist hegemony (control) requires the defeat of Communism, so that even very costly wars have a long-range rationale of making the world safe for corporate profit. This is an argument that can be wielded with great power, but it is not a universal explanation for foreign policy within capitalist nations. It should be noted that socialist states have hardly been immune from allowing economic considerations to influence their foreign policy either.