Renowned economist Paul Krugman this weekend laid out why he believes Donald Trump’s “weird” fixation on seizing Greenland had led to the president’s “stupidest” move yet.

In his latest Substack newsletter, Krugman noted that Trump has threatened tariffs against eight European nations that have sent troops to the semiautonomous Danish territory as a “tripwire” against U.S. troops potentially opening fire on allies.

Read the full analysis on Substack.

Trump’s tariff threat, Krugman argued, is “basically a howl of frustration on the part of a mad dictator who has just realized that he can’t send in the Marines.”

The Nobel Memorial Prize in Economic Sciences winner also pointed out that the targeted countries have previously struck trade deals with Trump and are now finding out that “a deal with Trump lasts until he feels like breaking it.”