In business, Donald Trump was a real-estate guy. In the presidency, he might be one, too. The prospective foreign policy of his second term has taken an unexpected turn, with his recent talk of annexing Canada, buying Greenland and taking back the Panama Canal. Rather than the neo-isolationist that he’s often accused of being, Trump is talking like a neo-imperialist, at least in our own hemisphere. Even if President Trump does none of these things — he’s largely joking about the Great White North — his musings are a reminder of the crucial importance of geography and the control of...