U.S. President Donald Trump is threatening to impose a 20 percent tariff on vehicles assembled in the European Union and shipped to the United States, in retaliation for European tariffs on American imports. On Friday, the day new EU tariffs went into effect, Trump tweeted, "...if these Tariffs and Barriers are not soon broken down and removed, we will be placing a 20% Tariff on all of their cars coming into the U.S. Build them here!" Auto industry experts say such tariffs could negatively impact the U.S. economy, as well as Europe's. "It's really a tangle; it's not a simple question" of cars being made in one place and sold in another, Kasper Peters, communications manager of ACEA, the European Automobile Manufacturers Association, said Friday in an interview with VOA.…