President Donald Trump's announcement that he was looking into buying Greenland, an autonomous territory of Denmark, was treated with derision by Danish politicians, Twitter users around the world (the idea was announced on the president's Twitter account), and by television comedians.
It turns out that the United States has tried to buy Greenland twice before. The U.S. State Department asked about a possible sale in 1867, and President Harry Truman made an offer after World War II. From a historical perspective Trump's impulse to expand American territory seems entirely consistent with the American story.
By the time historian Frederick Jackson Turner published his seminal essay entitled The Significance of the Frontier in American History in 1893, the American frontier had ceased to exist.