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In his now-famous speech in Davos last month, Prime Minister Mark Carney warned that middle powers must band together or risk ending up “on the menu.” His pitch rested on a simple proposition: middle powers, like Canada and India, which sit between a volatile United States and an assertive China, need one another. They should build enough connective tissue to avoid being isolated when the big players turn inward or hostile. It was a neat idea that will face its first serious test when Carney meets Prime Minister Narendra Modi on Monday, during his maiden official visit to India. The...