President Trump has lost no time asserting control over American trade policy, seemingly willing to levy tariffs on any partner at any time for any purpose.

He has threatened 25 percent tariffs on goods from Mexico and Canada, risking immediate disruption of the North American economy, and placed additional tariffs on imports from China, bringing tariffs on some Chinese goods to over 35 percent. On Monday, 25 percent tariffs on foreign steel and aluminum were announced.

To claim the authority to impose these new tariffs, Mr. Trump invoked the Trade Expansion Act and the International Emergency Economic Powers Act, two federal statutes that authorize the president to regulate international commerce during a national emergency or to address national security threats. In doing so, he sidestepped constitutional constraints on the executive branch to levy taxes without the consent of Congress.

But this was not simply a power grab. What Mr. Trump is doing with tariffs is a result of a lost consensus about how the United States should interact with other countries in the global economy. He is stepping into that vacuum, filling it with the unrestrained and autocratic use of import taxes, moves that appear to be based on personal whim rather than on U.S. trade law. Arbitrary trade policy of this kind reduces American influence abroad; harms working Americans, who often ultimately pay these import taxes; and makes the U.S. manufacturing sector less attractive to investors.

Since the passage of the Trade Act in 1974, Congress and the executive branch have, in effect, operated with the understanding that the president would be able to pursue a largely liberal trade policy to allow the free movement of goods and services between the United States and the rest of the world. Both parties recognized the value to national security of building and strengthening alliances through international commerce. Congress transferred its power to set trade policy to the executive branch in exchange for a commitment that these policies would cause no substantial injury to their constituents.

Rather than legislating tariffs line by line, Congress allowed for the president and his trade representatives to engage in negotiations over tariff levels and market access. Trade deals could be balanced with tariffs to offset economic injury caused by import surges, foreign subsidies and unfair competition.

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