a 28- memeber alliance led by the U.s

You seem to be referencing NATO, or the North Atlantic Treaty Organization, which — as of my last update in early 2023 — actually consisted of 30 member countries, not 28. It's an intergovernmental military alliance between North American and European countries. It was established on April 4, 1949, with the signing of the North Atlantic Treaty or Washington Treaty. The organization implements the North Atlantic Treaty that was signed on 4 April 1949. NATO's purpose is to guarantee the freedom and security of its members through political and military means. It promotes democratic values and enables members to consult and cooperate on defense and security-related issues to solve problems, build trust, and, in the long run, prevent conflict. The United States is a founding member of NATO and has been a leader within the alliance.