This essay considers the respective roles of the United Nations and the United States in a world of rising multipolarity and rising new (or old) Great Powers. It asks why UN collective security as a concept persists, despite the well-known failures, both practical and theoretical, and why it remains anchored to the UN Security Council. The persistence is owed, according to the essay, to the fact of a parallel US security guarantee that offers much of the world (in descending degrees starting with NATO and close US allies such as Japan, but even extending to non-allies and even enemies who benefit from a loose US hegemony in the global commons such as freedom of the seas (leaving aside pirates)) important security benefits not otherwise easi...