Andy Marsh’s New York factory is trapped in the Trump trade wars. As Mr. Trump threatens tariffs on America’s economic allies and its adversaries, many of the domestic businesses that the president says his policies are meant to protect are finding themselves victims of his aggressive approach. Prices are rising for imported goods, other nations are erecting retaliatory trade barriers, and companies like Plug Power, the manufacturing business that Mr. Marsh runs outside Albany, are facing crippling uncertainty from Mr. Trump’s fickle approach. It is not the first time Mr. Marsh has felt firsthand the impact of decisions made hundreds...