I believe most people don't assume that US (or other country, for that matter) is "truly altruistic" in your definition. However, the US is the first world powe

I believe most people don't assume that US (or other country, for that matter) is "truly altruistic" in your definition. However, the US is the first world power to make positive-sum policies the basis of its foreign relations (starting from the Marshall Plan onwards). You are right that this is not "selflessness", but positive-sum is even better as it is a sustainable approach.