I’ll let others describe the economic carnage President Trump’s tariffs have already begun to wreak. I want to describe the damage they will do to the American psyche and the American soul.

Trump is building walls. His trade policies obstruct not only the flow of goods but also the flow of ideas, contacts, technology and friendships as well. His immigration policies do the same. He assaults the institutions and communities most involved in international exchange: scientific researchers, universities, the diplomatic corps, foreign aid agencies and international alliances like NATO.

The essence of the Trump agenda might be: We don’t like those damn foreigners.

The problem is that great nations throughout the history of Western civilization have been crossroads nations. They have been places where people from all over met, exchanged ideas and came up with new ones together. In his book “Cities in Civilization,” Peter Hall looked at the most innovative places down through the centuries: Athens in the fifth century B.C., Florence in the 15th century, Vienna from the late 18th century to the eve of World War I, New York from the late 19th century to the mid-20th century, the Bay Area later on.

They were all meeting spots for people from different nations. Hall writes, “People meet, people talk, people listen to each other’s music and each other’s words, dance each other’s dances, take in each other’s thoughts. And so, by accidents of geography, sparks may be struck and something new come out of the encounter.” This, he continues, happens in junction points, places that encourage global interaction. Such places have common characteristics: They are unstuffy, un-classbound, nonhierarchical, informal.

Economic innovation explodes, he writes, “in places with a rich network of import channels, which in turn provide channels for new ideas.”

This used to be America. A crossroads nation, we attracted highly driven immigrants who wanted to be where the action was. We championed free trade. British colonialism and American internationalism made English the closest thing we have to a global language.