There's a huge debate going on right now about whether or not it is better to close all borders and become a self-sustaining country or to open up completely to free trade with foreign countries.
Trump seems to believe in an extreme form of protectionism which is worrisome. In the past such a self-sustaining fantasy was a disaster and was the reason in the first place to open up for external trade.
This policy has been the main reason that the United States has become such a powerful economic power today. Why our new president wants to tear this down and regress back to the era of blind fantasies is beyond me.
"... the well-meaning attempt to shelter inefficient local producers would always end up impoverishing a country, and hurting its most vulnerable members, because it would prevent labour and capital from playing to their strengths, would make goods far more expensive than they needed to be and would discourage leading companies from a necessary intensity of innovation."
Well actually, that's just one side of the story. For more enlightenment into the whole story, here's an interesting read: Free Trade - or Protectionism?