Elizabeth Cobbs Hoffman's American Umpire argues, according to the description of the book on the publisher's website, that the U.S. has acted as an umpire in the world, not an empire. The description says the U.S. has been willing to impose certain rules (which are enumerated in the paragraph) on the world. But then there is this sentence: "The nation has both upheld and violated the rules" (emphasis added). Well in that case, it hasn't been an umpire, has it?
P.s. The author has published an NYT op-ed which I haven't read yet.