As the EU calls Trump's threat to tariff allies for rejecting his bid for Greenland a mistake, Denmark's leader laments "being threatened by our closest ally."

By Philip Blenkinsop BRUSSELS, Jan 21 (Reuters) - U.S. customs authorities will face a real challenge if President Donald Trump goes through with his threat to place tariffs on six EU countries - rather than the whole European Union - given the ease of movement of goods between EU members.

Trump advisor Kevin Hassett calls for "cooler heads to prevail" as president threatens Denmark with tariffs over Greenland acquisition deal negotiations.