Donald Trump has a painting of Andrew Jackson in the Oval Office, but as he tries to browbeat Denmark into coughing up Greenland, maybe he should add a portrait of a Jackson acolyte -- James K. Polk.
If cold-eyed American expansionism is the theme, there are few better representatives than the 11th president.
He added more than 1 million square miles to U.S. territory and extended the country all th…