STOCKHOLM — As the term “middle powers” has gained currency this year, it has forced us to ask which countries actually belong to this grouping. After all, shouldn’t most states be described that way? With his much-cited speech at the World Economic Forum in Davos in January, Canadian Prime Minister Mark Carney revived a debate about shifting power structures in today’s world. He believes we are seeing a “rupture” in the global order, one that will require middle powers to cooperate to resist economic coercion and political domination by others. Although he did not mention the United States, President Donald Trump immediately sensed, correctly, that some of Carney’s remarks were directed against him. But, again, how should the middle powers that Carney had in mind be distinguished from the other powers of our time, if at all? Perhaps one should start by discussing those other powers. It was some time ago that “superpower” was applied to nation-states that wielded powers well beyond any others on the global stage. After World War II, this term was used to describe no