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.
But isn't that the case here? US didn't expect anything in return other than side effects of Europe being stronger (which was an advantage for them as well).
to enforce future battlefield so war could potentially be fought only far, far away and enemy would weaken itself, and US would benefit in similar fashion as from WWII... I am sure there were many aspects considered for Marshall plan, some could probably be called altruistic, but many could be called selfish.
>some could probably be called altruistic, but many could be called selfish
This is a zero-sum mentality in a sense. There are deals that benefit both sides, so there is no sense in selfish/altruistic dichotomy. One can be both "selfish" and "good" by pursuing positive-sum deals.
