The foriegn policy of the US has changed over time. Depending on what years you pick to compare, the changes may be more evolutionary than revolutionary, of course. But this is such an obvious point that I’m not sure how you disagree.
Are you familiar with the term “collective security” and the UN Charter’s general approach to it?
More specifically, lets say country A and B want to form some kind of security pact. Either a mutual defense treaty or some other arrangement. Do you think the UN is obliged to validate that pact?
Let’s say country B is attacked. Do you think country A is required to wait for UN approval to come to defense of B?